Amps should be sized to provide a commanding grip on speakers. This would translate into reaching 105 - 110 dB in your room (assuming you don't exceed the speaker's rating). 105 dB is peak for classical music (jazz would be less). 110 dB is average loudness at rock concerts. I recommend you play with a spl meter to learn how really loud these levels are. (Most audiophiles I've hung with average closer to 80 dB for critical listening.)
With your rather efficient speakers, that equates to 14 - 19 dB of gain at 4 ohms, or 11 - 16 dB of gain at 8 ohms (where most amps are rated) at the specified 1 meter distance using a single speaker. That translates into 13 - 40 watts at 8 ohms.
In general with two channels playing a 12 ft x 20 ft room, the room reflection (gain) and the limited distance you can sit away from the speakers the spl you'd measure corresponds roughly to the rated speaker efficiency. Smaller rooms would have a net gain, larger ones would have a net loss and so need more power. Heavily dampened rooms would see more of a loss.
Additional power is useful as it avoids clipping the amp, which is the easiest way to destroy speakers but, unless you want to break leases and ruin your ears, the chance of running your system that loud is fairly remote. This is more of a concern for users of tiny digital amps with average or low efficiency speakers.
I'd also be concerned with the 4 ohm speaker rating. Is that an average or minimum rating? Amps typically become unstable under low impedance loadings, however digital amps seem to thrive on low impedance loads.
Not suprisingly, digital versus tube versus solid state opinions vary primarily by what the vendor sells or the owner is currently using. Listen for yourself.
Also look into your source output sensitivity, your pre-amp input/output sensitivity, and the rated gain from your pre-amp when shopping for a power amp. It's a common problem to have way more signal coming into the amp than necessary (regardless of the amp's power rating). The result is limited usable range of your volume control. For instance I have a CDP with a typical 2.0 volt output, but the monoblocks have a rated input sensitivity (to provide full power) at 0.6 volt. The result is even with a passive (no gain) pre-amp I reach maximum volume at 10:00 on the volume control, so the volume can be somewhat hard to control. Having a pre-amp with gain would only make the situation worse.
