With the options of selling my M&K 350 thx and purchasing two LFM-1s or keeping it and adding the LFM-1 as a second sub. Sonically, is there a problem with using dissimilar subs? thanks

usually two of the same sub are used if you are going for dual. however if you can afford three subs, then you can have the one that is different as the lfe and the other two as powered subs for fronts. this would mean that the lfe sub would be the better one (especially for ht). i have used that setup in the past for some fronts that i liked but didnt have much bass, so i got some decently priced subs for the fronts to go with my other sub. this will do a lot more for you than two subs by themselves. i dont know what you have heard, but low frequencies are omnidirectional (you cant tell where they are coming from), there is a frequency (i forget what it is) that below that you cant tell where the bass is coming from. so in theory two subs handling lfe is not much better than one that is 2x better, if you catch my point. i.e. get one better sub rather than two lesser ones. i mentioned the using subs for the fronts to handle bass, albeit a little higher frequency bass that you would want to come from the fronts...

I don't agree entirely with your assessment, curegeorg. One of the major reasons to have 2 subs besides just bass reinforcement, is to even out the room response. Which two subs at good positions can do quite nicely.
Locating 1 sub is more difficult. But if you have one sub already at an optimum location, the addition of another can only help, unless it's located in a really dead zone.
You would probably want them in asymmetrical positions as not to chance further augmentation of any adverse room response. And being such low frequencies and basically non-directional, I don't think not being the same type is critical at all.
I hate to think of tb27272 giving up an M&K350, which a beaut of a sub, and I'm not sure he would gain that much with 3, for the added cost. Granted it would have more to do with the rest of his speaker system.

For just bass reinforcement, assuming the prime location is the best, one can even stack one on top of another for just 'more' bass. For that specific application, I think identical subs 'might' be warranted.
But in different room locations, I really doubt it will make a difference of any significance.
But I admit having limited hands-on experience in this area, and am basing it on where I have experimented with different room acoustics.

there is some validity to the room response/node, etc. i wouldnt put two different subs in pair to correct this though. the idea is to get a uniform sound, which you will not get from two different subs. i dont typically get into room dynamics, because its hard to give good advice without being in the room. very, very, few people have a perfectly planned out audio room, and if you are going to argue that another sub in pair will give you a flatter response, then why just stop at 2, you can get 4 and get even more of a flat response. you will get better performance from one better sub and a powered sub for each channel then you ever would from 2 lesser subs. that is why a lot of companies have powered towers everywhere, its just that the powered part of the tower is typically pretty bad. i would always buy one better sub than two lesser ones, unless i had an absolutely horrible acoustically neutral room. if i was thinking of adding some subs to complement one such as he has, i would not do so by buying one to put in pair with it, i would buy something along the lines of what i said previously. you can always spend more money on something, but that doesnt mean it is worth the money to do so, and you can spend a ton on audio, so i try to think of things in a cost effective sense. youd be happier buying two subs that cost the price of one and putting them up at the front, than putting the one in pair. that DOES NOT APPLY to the lfe sub though (as i stated above, one good is better than two bad), but when you are localizing the bass to a speaker its not as vital. i mean ideally youd put the best subs at all channels and lfe, but money is always a factor.... :-)

I still do not see how different subs would not be able to correct a problem, with regards to null and peaks in a room, unless the anomaly was occuring at point that was outside the range of one sub.
Ideally, with two subs, I would probably prefer to have one in front and one in back, or one along one side wall, but not centered.
And what is a 'better' sub? More dbs, smoother and/or more accurate response, or just one that costs more?
Although, I can certainly relate to having the one (if they are different) that will go deeper and put out more, handling LFE.
I am very interested in the subject, as I mentioned I would like to do the 'right thing' for my own setup, and would be interested in the experience of others also.

read this article:
www.harman.com/wp/pdf/Loudspeakers&RoomsPt3.pdf
there are also other parts besides part 3, just replace 3 with 2, etc.

I have asked the folks at SV Subwoofers a similar question. I asked about using two different subs of the same size and basic configuration (both 16-46 models, but with different amps, and drivers) their recommendation was not to mix subs, even units as similar as these. There concern was that it would be difficult to balance the outputs of the two dissimilar units. Since subwoofers are their business I have chosen to heed their recommendation. In short sell the sub you have, and buy two of the ones you want. Don’t mix.

its nice to get some support for a change :-)

Sorry, curegeorg if you thought I was not supporting you by asking questions.

I am not saying SVS is 'wrong' but they do make subs, so I might question their objectivity.

Anyway, thanks for the HK white papers which I have read before. The question of using different subs is not really addressed, although the other information is priceless, as always.

Here is a differing opinion on mixing subs, and though I think the answer is quite 'obvious' if you have read any of the information from the website before you would know the staff is very knowledgeable.

"Does one have to purchase the same subwoofer if they want to have two subwoofers (i.e., how would it sound if I purchase a M&K V75 {sealed subwoofer} with a Paradigm P-1000 {ported subwoofer})? Also, what do you think about the subwoofer external crossovers if I decide to purchase another subwoofer to complement my Paradigm P-1000?"

"Two different brand subs will not cause a problem, because low frequencies don't have an audible tonality. We use three different brands in the home theater test lab. As far as crossovers go, you may have to set each sub at a different low pass frequency if you run one from the LFE output on the AC-3 decoder and the other from the subwoofer output on the receiver, or if you run one with the rear speakers and the other with the front speakers."

here is the entire page:

http://www.hometheaterhifi.com/qa/qa9798/qa28.html

I would agree if SV were talking about different brands of subs. They were not. Their suggestion was not to use an SV 16-46 PC+, (a more expensive model) with an SV 16-46 PCi (the sub I currently have). In other words, buy another SV 16-46 PCi rather than buy the more expensive sub, (an SV 16-46 PC+) which might cause you problems. A company who was just out to make a sale might just try to sell you their more expensive model than give you proper advice. Go figure.