Just out of curiosity, what's the motivating factor for the upgrade? What are and aren't you happy with about the dinosaur (other than size)?
Short answer: I'm always trying to make my HT better. I'm hoping these subs will do that.
Long answer: 1994 build a new house. We decide we want one of those "home theaters" in the great room. Don't know anything about HT's, go to the store and come home with a Bose Acoustimass whatever. The great room has 18' ceilings, open sides, huge. You can imagine how great the base was. Go to a fancy AV store and explain the situation. Come home with the PowerField 1500. Stick it in the corner, it thumps pretty good, I'm happy.
Fall 1999: Decide to finish the basement so I can have a "real" dedicated HT. Not a lot smarter than I was in '94 but I do hire a local company that does the theater from the ground up. The finished HT room is 23 x 19 x 8.5. For speakers we decided on the NHT VT-3's that were coming out with VR-3 surrounds and VC-3 center. The finished theater sounds a hell of a lot better than my Acoustimass theater upstairs, I'm happy.
Over the last few years my HT has turned into a hobby. With the exception of the speakers, I've continually upgraded trying to make it a better theater. The VT-3 mains each have two 10" powered woofers. The base management is done by a separate VT controller. I have them sounding pretty good for both music and HT, but I'd have to say their base frequencies are more towards "polite" sounding. Six months ago I hauled the 1500 downstairs to see if I could make things less polite. The results were an improvement. More lower end impact, although I can't say mid base had the same improvement.
This is getting long sorry. Recently went to see the play "A Tuna Christmas". (Funny play.) Small theater, stage in the round. There's a scene where aliens land, (don't ask). Anyway when the space ship was coming down I heard something I hadn't heard/felt since the time 25 years ago when I stood behind two funny cars launching off the start line. Hard, tight, loud, in your chest, vibrating...etc. I'm trying to get some of that. In comparison it made my current setup sound duller, looser, not as articulate, less impact. In fairness to the 1500 and NHT, at the play, I probably was listening to a sound system costing over $100,000. Unless I want to spend that much on amps, speakers, or a top fuel funny car, I probably won't get there, but I'll keep working at it.
Keeping things in perspective, with my current setup any guest coming over would say it sounds great. Also, the unknown up till now has been exactly what the lower freqs have been doing in the room. The SMS-1 should help regardless of the subs I'm using.
The PowerField 1500 is 12 years old, so it will be interesting to see if sub technology has improved that much.