8w6 or 8w3 should probably work. You might want to check this datasheet on the Polk 8" to see if any other subs are a match. fwiw these polks are very very good for SQ, with 1.5" of xmax to xmax excursion they are pretty hard to max up and I'd imagine are better than an 8w3. An 8w7 just might work; it only wont work because the grill above the speaker only alows for about 3/4" of forward woofer movement before the woofer risks hitting the grill. I guess you could modify that with 1/4" shims.
http://www.polkaudio.com/downloads/m...er_MN1620D.pdf
I'm pushing these subs with a Soundstream Reference 1000sx (Which I bought cheap and repaired - HAHA!). These old-school amps Reference amps handle 1/2 ohm per channel and I've got these Polks bridged running paralel all the way where the amp is seeing 1ohm bridged. The amp claims 1000w in this condition. The gains are barely up; TONZ of headroom. This amp can handle what I want to do whenever I want to do it - and I want to go with 3x of these Polk audios next. BTW I'm sure this amp can absolutely rape these polks - but the great part is; the system plays so close to your ears in open space that you can hear any first signs of distress. Tune that junk out and power-up is just wonders for the headroom.
The other amp I have is a SoundStream MC140x which I also repaired. its rated for 35w x 4 which is fun - best part is that its actually better sounding than every other amp I've had on my workbench - Thanks Nelson pass for the prototype design!