Plugging powered speakers in....??

Seeing that we're only talking about BASS, and not midrange/tweeter, you're fine plugging in the speakers to your outlet for best sound! There's really not going to be any significant improvement going into any filter or conditioner. INfact, you'll likely only LIMIT DYNAMICS!!!
Also, there should be a fuse for the amps in the subs, and I personally wouldn't sweat the surge protection any.
Worst case scenario, you had a lightening storm strike, and you blew the amp on your woofers. NO biggie...I'm sure it would be taken care of by manufacturer

I have my powered sub plugged, along with everthing, into my Audio Magic Stealth. Don't neglect your sub. The difference, while suble in the beginning, becomes much more evident through time.

Monster makes a filter just for subs that plugs into your wall outlet. I think the model number is SW200. I think this should be the minimum you consider unless you want a power surge or lightning strike to take out your powered subs.

EVERY SINGLE LINE CONDITIONER, OR LINE FILER, OR POWERSTRIP I've ever come across in this business has either limited dynamics in the bass reigion, or been neutral there basically!....I'm sure some would argue. And, at best, those things are system dependent it seems. And, although I am an advocate of using line conditioners,filters, power supplies, etc, on a large varriety of systems I do, I would not recommmend it in this case.
That said, seeing that bass is the single most room/seating/speaker placement dependent factor in an audio system, and given the fact that 99+% of ALL audio enthusiests and audiophiles are going to have bass that's so far off from "flat frequency response" and coherence anyway that it won't matter, I would say that A POWER LINE FILTER for your bass IS NOT GOING to make a beans worth of significant differnce in terms of performance for your powered sub's improvement!!!!...IME
However, to apease your curriosity, my suggestion is to TRY OUT SOME LINE CONDITIONERS YOURSELF, and do some A/B'ing! I would bet you you'll end up deciding it doesn't make any real difference in terms of bass performance in your system.
good luck