How much power is to much power?

While we're waiting for some of Audiogon's more technically astute members to respond, I can tell you that based on my experiences, the 91db specification doesn't tell the whole story when determining amplifier compatibility. Your speakers impedance needs to be taken into consideration too. That information can be hard to get. The manufacturer will probably give you a nominal and minimum resistance value but a plot covering the speaker's entire frequency range would be much more useful. FWIW, my current speakers are rated slightly higher than 91db and I couldn't be happier that I moved up from 70W to 200W amps.

Very difficult to have 'too much power'....But why bother?
You have fairly hi sensitivity speakers. 50 or 60 hi-quality watts should do the trick, unless you want to induce ear bleeds.

My speakers are 92 db efficient and I use 700 wpc monoblocks. I do not listen loud, but these speakers have a notoriously difficult load and need a lot of current.

The SS amps that you are comparing may not be different or the 140 wpc may have more actial power. Check out Levinson 33H "150 wpc" amps as an extreme example.

What amps are you looking at and what speakers do you have and how big is your room.

I'm using 1000 watts into 4 ohms for my 91dB speakers. It's not a matter of speaker sensitivity or loudness, it's a matter of controlling the drivers. Speakers like Vienna Acoustics, B&W, Wilson, Dali and others have the best overall and bass response when the amp really damps and controls the large drivers, so, with these type speakers, generally more power will equal more control and better sound.

OTOH, some speakers are so efficient, approaching 100dB that you can drive the speaker with 10 watts and really control it with less than 100 watts.

What speakers are you driving?

Dave

Reynaud Twin Signatures, will possibly upgrade to Reynaud Offrande in the not to distant future. The amps in question are vintage Accuphase c-200/p-200.