new Parasound A21+ vs my old Rotel RB-1080

Again, don’t listen to some guy remitting biased opinion / exaggeration. Look at what he said before (see below), just opposite to what Stereophile has commented. A lot of people misinterpret brightness as being ’detailed,’ and I doubt some guys’ opinion is well-founded. Go look at the reliable reviews on ELS-X, no one has raise this issue.  I also owned ML Aerius in 1996 for a short while and found no issue like this.  I sold it because my 1 year-old son continuously trying to polk holes on the panel.

because the Rotel is already a pretty detailed amp in the treble.

Saying that Parasound is a step down from Rotel is absolutely untrue—it’s an overgeneralized statement. To be fair, Parasound's high-end models are a step up from Rotel’s lower-end models, while Parasound's lower-end models are a step down from Rotel’s higher-end models. As a satisfied owner of several Parasound amps and preamps, along with many other users here, I can attest to this, supported by cross-references with reliable reviews from prestigious audio journals covering both brands.

Don’t listen to someone’s semi-educated guess—especially if they have never owned or auditioned this particular model of Rotel. You need to do your due diligence. According to Stereophile:

But I’d be misleading you if I gave the impression that one would ultimately want to get by by using the Rotel RB 1080 to drive the Revels, or even the B&Ws. The 1080 was not the last word in soundstage openness or grain-free treble, and always retained a slightly withdrawn mid- to upper treble. Switching swiftly between power amps in less than a minute, it became apparent that amps such as the Simaudio W-5, the Bel Canto eVo, and the Sonic Frontiers Power-3 did not suffer from the 1080’s minor foibles in low-level micro-dynamics and treble reserve. Furthermore, those other amps ably demonstrated that the Revel and the B&W are capable of truly transparent treble, detailed and finely drawn, which the RB 1080 merely outlined. On Cyndee Peters’ "House of the Rising Sun" (Test CD 4, Opus3 CD 19420), the ultimate delicacy of the triangle and brushed cymbals eluded the Rotel but not the other three amps.