Mike -

Sorry - I assumed you knew the NHT 1.5's are monitor style speakers.

As far as how much improvement, the Yamaha has a 90 hz cutoff, for all speakers, in the speaker selection, which I think is fairly high. The ICBM has 40-60-80-100-120 set points for all channels. The 1.5's have a -3db response down to about 53 hz. The AC-1 has a -3 db response of about 75 hz. The SubOne has an infinitely variable low pass crosover, so that's not an issue. If I were to set all crossovers at 80 hz, there would likely be a small change in the sound compared to the Yamaha fixed 90 hz setting.

My issues are around integrating bookshelf style speakers and a subwoofer, for both music and HT. I can tell you that changing the crossover point on an ICBM channel makes a substantial changes in the sound. I'm still tinkering between 60 and 80 for the mains.

Depending upon the controls of your sub, the ICBM can offer add'l flexibility. There is a front panel control for both sub volume and LFE output, so you can adjust both the level of all signals sent to the sub, and also the mix of LFE in the mix of signals sent to the sub.

The ICBM also has sub crossover slopes of 12 db and 36 db. Again, a marked difference in the sound at each setting.

So - To try to answer your question - the amount of change with an ICBM can be subtle or very marked, depending upon the settings you select. I would think that with your speakers, you would have an an opportunity to tinker with the 40 hz setting for your mains, and send only true sub-base to your sub. However, you may like the "cleaner" sound of your mains if you set the crossover at 60 or even 80, so that your mains woofers are only producing mid-bass.

Again, not tring to be evasive, but you just won't know till you try it.

Hope this helps. Good luck!