"90dB", by itself, is not a useful specification. It could mean anything.
It should be referenced to power applied (eg 1W) and the distance the measurement was taken (eg 1m).
If the true specification is 90dB/1W/1m then it is a somewhat unusually efficient speaker and a 100 watt per channel/8ohm amp is more than adequate.
If those vital qualifiers are not there from Polk, I suggest you find a review where they measured the sensitivity of the speaker, or eMail Polk and ask them directly, and use that value.
I assumed you meant the AVM-2 Power Amplifier from Anthem; they've never made a PVA-2 amp.
I used the value of 100 watts into 8 ohms because the quick search I did showed only one review with an actual test of the amp, and they measured 119 watts into 8 ohms with a 1Khz signal and 1% THD. The 4 ohm power under the same test conditions showed 193watts. There is no indication if a burn-in was used, and no indication if this was with just one or both channels driven, even though I read through the page on the test method they use.
That is a fairly undemanding and uninformative testing method, and normally I would reject it and find better ones. However, there are not a lot of lab tests for this particular amplifier available, so you use what you can get.
The FTC Rule says power for a 2-channel amplifier must be specified as watts/channel/all channels driven/8ohms/ from 20 to 20,000 Hz/specified distortion level, measured only after a 1-hour warmup period at 50% power, which stresses the amplifier enough to learn if the protection circuit will kick in when pushed.
From a look at Anthem's website, they rate the amplifier at 105 Wpc into 8 ohms via the FTC method, so my guess was pretty close there. However, I see that they rate it at only 140 Wpc at 4 ohms, so I was a bit generous with my estimation.
Multichannel amps are generally tested at a much less stringent test regimen. Anthem officially refers to the AVM-2 as a multichannel amp, presumably because it can also be ordered in a 5 and 7 channel configuration, so they may not use the FTC Rule power method (which you must use on any amp called a "stereo" amplifier, by law), opting instead for the much easier multichannel method, in product advertising. That would explain the 125 watt rating, at least.
It should be noted as well that there is really no standard method used for measuring multichannel amps, although most will specify the method they do use. A test frequency of 1Khz is the easiest load for any amplifier to drive, and will almost always give a maximum power rating higher at that frequency than at 20 Hz (bass region) or 20Khz (high treble region). Multichannel amps are often rated with only one or a few of the available channels driven to maximum power, and usually at a 1Khz test frequency.
With only 1 of the two channels being driven, Anthem rates the amp at 125 watts/8ohms, but they do rate this level of power from 20Hz to 20Khz, which is good.
This is probably so you can compare to other multichannel amps from other manufacturers, which generally use a very, shall we say optimistic, power test procedure.
But, if you're comparing stereo amps, it's a 105 watt per channel amp.
I liked all the Anthem amps I've heard. As long as it's working when you buy it the age isn't an issue. 7-8 years old is relatively new.
