Most subs seem too powerful

Subwoofer frequencies travel far and are difficult to absorb - sorry but you can't change physics.

With subs, setup, room placement, hookup option, adjustment, are all critical. You might want to do some research on that. You may find that tweaking those adjustments are all you need. Most folks tend to set them up on the overkill side, which tends to sound boomy. I don't know what you mean by turning up the volume to 30 or 40 watts. I use 8 watt SET amps in a modestly sized room in nearfield listening. I use an ACI Force XL and am quite happy with the way I've been able to integrate it with my Silverline Sonatina speakers. Do some searching on proper subwoofer setup. There's material here in the forums, as well as on manufacturers sites. Shardone makes a good point; if you add those low frequencies that are not currently there, they will travel and your neighbors may not be happy. I find the bass frequencies a bit less 'rewarding' at lower volumes myself and if I were limited on volume I would probably not bother with a sub myself. That's just a personal preference though.

Good luck.

It's not just the volume, it's the frequencies they reproduce. You can have the thing turned down but the bass frequencies will still resonate other objects that have the same resonant frequency. If you're in a wood frame building that you can't structurally alter, or build bass traps for, the problem will not go away.

I have a REL sitting on an ASC SubTrap (http://www.asc-hifi.com/sub-trap.htm). I live in a wood frame apartment, carpeted floor, and the REL augments a pair of Silverline SR15 m0nitors. In elevating the sub, I found the bass to become more airy, more integrated at nearly the height of the monitors.
Cleaner too, and less resonance. The differences from sitting the REL on the floor are very rewarding IMHO.