I have no clue what the dB settings on my Denon mean. If anyone has a good explanation, I'd love to hear it.
To get to Troy's issue, at sub-woofer frequencies (30Hz and below), the M80 requires about 1W to produce 66dB at 3 feet (at least in my room it does). So for the pair, you can get 69dB at 3 feet per watt. If you're sitting 8 feet away, you may still be getting around 69dB if your room is quite live. 69dB isn't very entertaining so you probably have it turned up to say 84dB so now you are consuming 32W for those lows. Once you add a sub, your amp will be relieved of these 32W.
More importantly, you have the main volume turned up so high in order to hear the lows that you are torturing you and your loved ones with the mids and highs. Since the 80s are more efficient with mids and highs, you are probably pushing more than 105dBs in those ranges and I am sure you are taxing your amp during dynamic passages.
So when you add the sub, you will have more sane listening levels and it may even sound cleaner too.
