Is using a X.1 AV receiver just for stereo bad?

the answer, unsurprisingly, is it depends on the avr. i was very pleased using a b&k avr 202 and 307 for two channel stereo; i believe an arcam avr (e.g. the 305) would sound even better. correspondingly, my quite pricy denon avrs just don't hack it; likewise various marantz i've cycled through.
i agree that if you don't need hdmi and other such foolishness, there's some vg values in high-quality used avrs; purists will argue that separates ultimately sound better, but there's some real advantages (budgetary, convenience and otherwise) in your proposed approach.

Zanon, most people believe that (all things being equal) for a host of engineering reasons separates will sound better than comparable integrated components, although of course a high-quality integrated will outperform suppar seperates. The ultimate question, as you infer, is whether the sonic differences are cost-effective. Now, I often see older Arcam and B&K AVRs on Agon and Ebay for as little as $200. These AVRs not gonna sound as good as a system consisting of used Arcam or B&K componenents and an external DAC, but they'll probably sound 80% as good for, say, 25% of the price. Whether the improvement in sound is worth the extra $$ is, of course, the dilemna we all must face.

i'm not a techie, but looks like depending on model they use class a or a/b designs; b&k appears to use class d on some separate amps. the more i read, the more i'm impressed with their engineering--these mfrs. absolutely blow away the asian crap in terms of design and build quality.