I am installing a single Polk MM2084 8" 4-ohm sub due to space constraints. It is rated at 30-200w rms. Every mono amp I see puts out way beyond this power range. What would you recommend for an amp to power this speaker? Thanks
Find a small bridgeable stereo amp, rated arond 35 to 50 watts per channel (not more than 75). Bridged, this would provide you with very close to your desired 200 watts.
In fact, if interested, I have a small Eclipse amp (the 3122) that I'd be willing to sell you for cheap. NEVER abused, (ask anybody here that has bought anything from me - my stuff is ALWAYS super babied) it is still very clean, and actually installed in my system right now, doing nothing! Rated at 55w RMS, it'll bridge perfectly to your one woofer. Let me know.
BTW,
here's a review on that 3122. In fact, I'm pretty positive I still have the box it came in, with installation manuals and (should be) all the packaging.
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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
And if that doesn't work I've got an older Kenwood mono amp, around 140w I think hanging around the basement. KAC-716, has Xover & takes speaker or RCA inputs.
You're right, there's a trend towards ridiculous power levels these days. You can still find "normal" amps tho, just check for their RMS power into 4 ohms, that 1000w watt amp will rapidly become 150w.
Whoa, check this out:
Buttkicker subwoofer amplifier at Parts Express
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
I am unfamiliar with the Buttkicker. How do they do for powering subs?
Well come on, it's called "Buttkicker" what more do you want??? 
Seriously, this company produces some nice tactile LF systems for home theater. HERE's a link to the product this amplifier was designed for. It should be a fairly decent quality amp, and at PE's price it's worth a shot.
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Support the12volt.comI had a similar need for a sub amplifier a couple years ago, needed about 200 watts. Not much available, like you are experiencing. I found fortune, though, in finding this USacoustics amp available at a discontinued price of about $135 from Crutchfield. Over time, this little amp has been a cool gem, but finding info about it that is accurate has proven to be a crapshoot. The USacoustics website shows specs that are dissimiliar, in some respects, to the specs printed in the owners manual that came with the unit. It shows 250 watts output, but if you look on down the page to the fuses, there is only one 20 amp fuse. The amp truly has only one 20 amp fuse. A quick calculation using Ohm's Law and giving the amp the very best scenario of class D efficiency proves that it cannot possibly perform at higher than 200 watts whether the load is 4 ohm or 2 ohm. Just with that, though, the amp has given my system all the sub power it wants.
I thought about this amp when I read your post and then later saw this special at Parts Express (where I like to shop for specials from time to time.) I knew about the Buttkicker products from reading the AVS home theater forums. There is a theater seating manufacturer that will preinstall these tactile transducers in the seating when you order them. I found that a lot of higher end HTs use Buttkicker (regardless of its name!) transducers and amplifiers.
So, why is this amp okay to use with a subwoofer when it was intended to power a low frequency transducer? The LF unit uses sub frequencies too, only they are lower than what a sub will play. But the amp also produces power for the full subwoofer range, as well. What this gives you, after all is said and done, is a sub amp whose capabilities exceed that of a normal (comparable power) subwoofer amp. Just with the links I provided in this post, compare the specs: damping factor, THD and frequency response. The Buttkicker shows much better specs. Also note the fuses in the pic; the Buttkicker employs two 20 amp fuses, which will allow for its boasted output abilities (just as my USacoustics did not, with its one 20 amp fuse). It also has a variable LP filter, selectable 20 Hz SS filter (for vented subs) and variable phase adjustment. My amp has a 180 degree switch and no SS filter.
The price will make these units go fast; I've seen this type of thing before. A lot of people in the home audio and home theater market will take a hard look at these. The only thing I'd consider doing, if it were me, is repaint over that crappy "Silent Subwoofer" lettering emblazoned over the top of this unit!
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
Thanks for all the help. I ordered a Buttkicker (sorry, the name cracks me up) and will report back with results.