Sorry, I can't help with the first part of your question though I suspect, if I read the question correctly, you'll have to try the Marantz to determine whether it is "better" than your present set up. But I'm confused why you have duplicated or want to duplicate so many functions in your system. You have two headphone outputs, two phono pre amps and essentially two amplifiers for driving headphones. And now you're thinking about adding another "pre amp" and another headphone amp? Ay chijuajua!!!

Rgearding your second question, you haven't done enough troubleshooting to allow for a really good answer. There's always the possibility the distortion is inherent in the disc or is the result of improper set up in your turntable. If I assume all your other material sounds OK when played on the same set up, then I would say your problem is possibly one of gain/output voltage and overload sensitivity between your phono section and the Marantz integrated. When you turn the gain up on your phono pre amp you are increasing the voltage input to the Marantz and it likely doesn't care for the excessive amount of output voltage the Bellari is producing.

However, that doesn't assure you the phono pre amp is not the source of the distortion. Typically, a volume control will be placed before the output gain stage of a device which would mean you are lowering the input voltage to the tube by lowering the "output" level. I have no idea how specifically the Bellari is configured so there is no way to make a good guess whether this could be the issue in your system. Assuming that it is, a clean sound when the volume control is lowered might mean lower input votage to the tube is preventing the tube from being overdriven while the higher gain setting means the tube is being driven into distortion and merely passing that to the Marantz.

From what you've posted I can only assume the speakers are not distorting under the same conditions as the headphones. That could mean you are simply overdriving the headphone amplifier.

You have sufficient versatility in this system to do more troubleshooting. From what I see and read you have two phono pre amps available to you and two headphone outputs along with the ability to separate the main power amp from the pre amp of the Marantz. Driving the Marantz power amp with the Bellari line oputput would be another option to try as you sort out where the distortion occurs. Ignore where the volume control sits, 10 O'Clock or 5 O'Clock doesn't matter. If you have sufficient volume level, that's all that counts.

You don't have to be extremely well versed in how amplifiers work to do some deeper searches into how your system is operating. All it takes is a logical approach to ins and outs and a bit of an understanding of what might be causing problems. Typically you begin with the simplest system set up possible and add or subtract components until something tips you off that you are on the right or wrong track. Take notes if necessary and follow a logical path to how you now have the system running. When you find a connection that causes the problem or makes the problem disappear, you have to stop and think about why that would occur.

If you are running the Bellari into the line level inputs of the Marantz - you're not running it into the phono inputs I suspect, you might try taking the outboard phono section's output into the "tape monitor" inputs. That input might have a higher overload sensitivity and this might solve the problem. But until you do more searching on your own, that's about the best I can suggest.