Help me solve a mystery

OK, I'm a bit perplexed. I recently bought an Audiomat Prelude Reference here on the 'Gon from a very reputable seller. Received it in great shape, connected it and it sounded glorious! Beautiful, wonderful, incisive. Everything I had read about in spades.

I then decided to swap out some interconnects to find what was going to have the best synergy in my system. When I tried the first swap all of a sudden my right channel was almost completely gone. And it was gone with my digital front end as well as my vinyl setup (this is where I started with cable swapping). Swapped the ICs back - same thing. Still no right channel.

So my first thought was bad tubes in the new amp. I swapped the input tubes with an alternate I happened to have on hand- no change. And then I swapped the EL34 power tubes (left pair of tubes going on the right side and vice versa) - still no change!  Then I thought maybe it's something else in the amp, so after I put the tubes back in their original places I swapped speaker leads so the amp's right channel was driving my left speaker and vice versa. After doing this my right channel was still out, which made conclude the amp was fine and that maybe something later in the chain was causing problems.

At this point I should add that the more closely I paid attention I noticed that the right channel was there at very low volumes but when I started inching up the volume it didn't increase equally to the left and actually seemed to recede as I applied more juice.

So then my inquiry turned to my speakers. I have Living Voice OBX and to make a long store somewhat shorter I swapped crossovers, then speakers, moving them from one side of the room to the other, and in every situation the left channel was strong and the right channel was week or non existent. As a last ditch idea since my speakers are 6 ohm nominal I decided to move the speaker cables from the 4 ohm to 8 ohm and voila! The right channel came back and it sounded wonderful again. Perhaps something was going on with the transformers??? 

But it sounded so good on 8ohm taps that I decided to leave it for now. But I did want to reorganize my cables since they were an absolute mess behind the stand and I also wanted to experiment with taking out one of the two power filters I had, and connect my sub, which I had not done yet.

Soooo.... I slid the rack out and took one power filter out and reworked how everything was powered to accommodate one less conditioner. I connected also the sub but did not change the way the speakers were connected to the 8 ohm taps, or change ICs. I then carefully slid the rack back in place and was ready to return to glory.

Powered up and guess what? Right channel is gone again! 

I am at a loss and could use some expertise from smarter minds than mine. The only think I haven't done in the amp that I could do is replace the one driver tube that sits in front of the two input tubes. I don't have a replacement on hand but could order one. I understand it's quite old but for the life of me it doesn't make sense that if it were the culprit I would be getting the same problem with the right speaker, regardless of how and where the speakers are connected. But maybe I don't understand something in the way the circuit works.

Could it be transformers? If so, I'm guessing I'll need to send them into Audiomat for repair/replacement?

Could it be something else I haven't tried? I don't know. Any helpful thoughts would be quite welcome here.