Amp / Tubes / Channel

When we moved in together, my partner introduced me to her trusty old friends - a Golden Tube SE-40 amp and an SEP-1 pre-amp (plus a pair of small, but impressive Optimus speakers).
I feel like I'm breaking commandments by connecting my 20-year-old magnavox CD changer to these classic engines, but I forged ahead, including new cables all around. The improvement over the sound that came out of my old components is unbelievable.
However, only one channel plays... I have two questions. Could the lack of signal on one channel be tube-related or is that kind of problem likely to be internal? And would replacing all the tubes make a big difference in sound? (tube recommendations?)
First are you sure it is the amp? Have you tried any other sources? Have you tried things like taking the cable from the side that doesn't work and put it in the side that works? Moving the tubes from the right side to the left and check bias.

As far as tubes they will make a difference. Big difference - Maybe. Either for the better or the worse. Usually better. Some tubes are warmer sounding than others and some are very linear. The only way to know is to try them. Since I'm not familiar with that amp I will not make any recommendations. My Octave V70SE takes many different types and I will say different brands of the same type will be different sounding.

Good luck
Reverse all of the tubes. Right clannel to Left and left to right. If the other is now dead, you have a tube issue.
Thank you for the replies (yes, tried switching cables, etc, and my old receiver drove both speakers, except it was my old receiver...)
Next step, move the tubes, reverse them - I guess that's too logical for me to have figured out for myself. My upbringing was solid-state. I see that all the tubes glow in the dark but that's about my limit.

Thanks for your wisdom. I'll post the outcome.
I moved the tubes - reversed them. Still, the same one channel plays and the other does not.
I need someone knowledgeable and qualified to get inside this amp. I live in Rochester, NY. Do you have any recommendations? Re: Repair of Golden Tube amplifier?
Reading through this post, I'm not clear if you tested all of your components to make sure that the problem is with the amp. For example, if the right channel is OK and the left is not working, go to the back of your preamp and unplug the cable for the right channel of your CD player. Then take the CD players left channel and plug it into the right on the back of the preamp. Play a CD and if you still hear music, you just confirmed that both channels on your CD player work and its definitely OK. You need to do this with all of your components (if you haven't already).
Bf. Call Joe at Service Technology at 585-427-2970. He been repairing hifi for over 40 years. He fixed my sons tube headphone amp and also does repairs for Rowe I believe. He's over on Brighton Henrietta Townline Rd near Winton.
Thank you