Having become a Sunfire owner prior to the availability of the LFM-1, I can comment as follows:

Just about any reasonably performing sub can be improperly blended in a system and sound “boomy,” including the Sunfire. Just about any reasonably performing sub can be properly adjusted to blend well and make an awesome contribution to the system’s sound without drawing attention to itself.

Regarding low-end performance, I can send sub-sonic signals to a Sunfire and watch the drivers moving without hearing any direct sound, but then some things in the room rattle on shelves and my pant legs are flexing because of the waves of changing pressure traveling through the room from several feet away. Haven’t found a bargain sub that manages this. Is this type of low-end extension necessary for music or HT?

Depends on your goals, but IMHO, very rarely.

When might it make a difference? There might be audible frequencies in some recordings that interact and some portion of the sum or difference between the interacting frequencies results in inaudible frequencies that add to the overall experience. There can be source material where some very low frequencies are enhanced or created and inserted for musical or other effect. How much would this bit extra of performance enhance your experience?

Is it worth a doubling or more of the expense if budget is a concern? Not in my opinion. In a subsequent subwoofer purchase, I tried two subs that were each ¼ of the cost of a single Sunfire. I found that their performance was reasonably good when I’m not looking for that nth degree of low end.

So having been both a Sunfire customer and a bargain-sub-with-decent-specs-but-not-a-lot-of-umph customer, and having been satisfied with both ends of the spectrum with respective goals, I would not push the financial envelope in order to exceed the capability of subs in the LFM-1 class. The question becomes, how big is your envelope?