@angelgz2
Yes you should keep your Parasound Halo A51 amp. The Marantz, Anthem, Arcam, Audio Control, McIntosh, Yamaha, Onkyo / Integra HT processors are early adaptors and especially the Denon Marantz Onkyo/Integra are usually the first that adapt new AV technologies and new formats & codecs.
The new Marantz AV8805 will perform and sound better than your Rotel RSP-1576 processor. Rotel makes great amplifiers but when it comes to HT processors IMO the Marantz is slightly better. Besides, your Rotel RSP-1576 is not a Rotel’s top of the line HT processor. Currently the RSP-1582 is Rotel’s top of line HT processor but the RSP-1582 does not support latest formats and codecs eg Dolby Atmos DTS-X Auro 3-D, 4K HDR Dolby Vision. The RSP-1582 sounds and performs better than your RSP-1576.
I got a feeling that Rotel is probably going to release a new replacement for its flagship RSP-1582 possibly next year.
In regards to a tube power amp vs your Parasound Halo A51 driving your ML electrostats, I don't think you will be able to find a five-channel tube power amp. Or unless you're thinking to use a stereo or monoblock tube amps to drive your front ML electrostats? What kind of ML electrostats do you have? The big ML electrostats would need big high current solid state amp to drive them properly. Their speaker impedances are very low and can drop to around 2 ohms or below. So a low wattage tube amps just won't cut it, unless if you get a hybrid amp (tube input stage & solid state output stage) such as the PS Audio BHK 300 monoblock amps or the BHK 250 stereo amp.
