Not sure how a 2-ch stereo preamp with a HT Bypass works. But ideally I
should be able to connect Oppo’s Stereo XLR/ RCA for music & 5 out
of 7.1 RCA Audio for movies to my Parasound A51.
You don't actually need a HT bypass switch. You can use any preamp with a home theater processor. If there is no HT bypass you simply set the volume control to a particular position and let the HT processor handle the rest.
Since the Oppo and your amp are balanced, it makes sense to use a balanced preamp. I don't agree with Tim's assessment above though- a good tube preamp will give you a more neutral/natural presentation than solid state and will be less irritating in the high frequencies. This is because solid state in general has higher ordered harmonic distortion that is more audible than it is with tube gear. I don't like the 2nd harmonic 'warmth' coloration that is often blamed on tubes but if your preamp is fully differential and balanced that harmonic will be absent. In general though tube preamps have far less of that sort of thing than amps so its not a big deal.
What might be a bigger deal is how the preamp handles balanced lines. There is a standard for balanced operation called AES48 (Audio Engineering Society file 48, although the standard was around long before the AES itself). Very few home preamps with balanced operation (tube or solid state) support this standard! The standard is quite useful as the interconnects used with the preamp won't impose any sonic footprint (that is part of the reason the standard was devised)- you won't have to audition cables to get the system to sound right and be neutral. When the gear does not support the standard all bets are off. So its a good idea to check with the manufacturer to see if their preamp supports the standard. When asked, it should be simply 'yes' or 'no'; if the answer gets much longer than that there's a pretty good chance that the manufacturer doesn't support the standard and may not even know what you are talking about (FWIW our gear does support the standard). One of the aspects of the standard is the input impedance of your amplifier will not be an issue- any device that supports the balanced line standard should be able to drive a low impedance line which might be only 600 ohms, so the typical 10K to 47K input impedance of most solid state amps will be a breeze.
If the preamp you select employs an output transformer then you will have to pay attention to loading the transformer for optimal bandwidth. I mention this because in the balanced standard, the output of the preamp floats with respect to ground; each side of the output signal (pins 2 and 3 of the XLR) reference each other to complete the circuit rather than ground. This is done to prevent ground loop problems. A transformer is one way this is accomplished; the only other way I know of is the method we patented, which is direct-coupled.
