Thanks everyone for the passionate and detailed responses!
It looks like both of the 2 problems mentioned (room setup and the amp) are indeed there. When I listen to the speakers close-up, the sound is much finer then when I position myself in the "sweet spot" not to mention any deviations from that...
To geekhd: Yes, I've seen this response curve; actually, I might draw the similar one having my ears as the only measuring instrument; yes, high freq. are fading and there is a boomy peak near ~100 hz. Positioning the speakers differently does make difference but not resolves the problem.
I do consider EQ and I've already tried my DSP-1124P (set manually, as a first approximation; I know it's crazy to set parametirc EQ manually). While the frequency response became OK, some finer structure of the sound disappeared (and my wife can hear this as well). So, I am thinking about some old good analog EQ (Yammi EQ-70?) According to the specs, it should be transparent. I've learned about the Room EQ Wizard on this forum and I do have Behringer ECM800 individually calibrated microphone (just had no chance to start all of this yet).
Why I started to ask about amps? Everyone says that RX-797 is a good pre, and I question this statement because (oh, this sensitivity will kill my budget!) I easily hear the difference between "pure direct" and "CD direct" modes and easily hear the difference between the headphones amp in 797 and the above-mentioned Xenix dedicated headphone amp. And (correct me if I am wrong), an external EQ may be hooked to the RX-797 in "pure direct" but non in "CD direct" mode (I don't have any schemata for 797 but the CD direct seems to bypass anything hooked to a pre-amp out)!!! And once you listen to CD direct, there is no way back, and I started to suspect that I don't want certain circuits on the 797's preamp... This makes me think about a finer amp. Could anyone comment on the two I mentioned in the original message?
ap1: "...Behringer DEQ2496 (http://www.behringer.com/EN/Products/DEQ2496.aspx
). You probably will do better if feed it with digital output from your player."
Thank you for the link! May I hope that this piece of equipment won't "shave out" the fine structure of the music like 1124P does? Your suggestion to use the digital output from Oppo SE is an interesting one... Does it mean that AD/DA converters of the DEQ2496 are so fine that we can easily forget about the fancy Oppo's SE DACs?