Actually, it's better to lower digital volume slightly no matter what. A full 16 bits of resolution is usually unnecessary anyway, and all DACs perform worse with 0 dBFS signals.
Digital volume control has no channel balance issues, and does not increase electrical noise, distortion or crosstalk. It's much better than analog volume control unless too much is used and is making the quantization noise audible. -10 dB from ReplayGain is almost guaranteed to not make this happen, and absolutely guaranteed to not make this happen if using 24 bit playback. I use around -20 dB of digital volume control myself, between ReplayGain, my EQ, and my soundcard's volume control.
As long as you aren't lowering it enough for the quantization noise to be audible, there's no problems. If you're listening in an anechoic chamber at 90 dB with a 16 bit signal, then yes digital volume control like ReplayGain may make the sound worse. If you're padding your 16 bit files with 8 extra bits during playback, using a 24 bit signal, you have -48 dB of digital gain to play with before you lose any resolution at all, let alone audible resolution.
No recorded music in existence has enough dynamic range to make -10 dB compress it, the highest dynamic range I've heard of is 65 dB and most tracks are around 5-15. Dynamic songs are still ReplayGain's weakness, but that's because if it boosts the volume too much it can compress peaks to prevent clipping. It's the positive gain that's the problem, and is pretty much applicable to only classical. This problem can be solved with the preamp, or more digital volume control somewhere else.
