Old tube amp sounds good only at full volume

[disorder]

This isn't a guitar amp but it's a pretty simple 5w hi fi amp. It's a Voice of Music 162 that powers on and passes signal but only sounds good with the volume dimed and the input signal as high as can be. I'm feeding it music from my MP3 player. It sounds great like this but anything below full volume (or if I decrease the volume coming from my ipod) it starts to get very gated. Crackly, gated, choppy

Here's the schematic.

Input to grid of 6AU6 via a potentiometer. All resistors measure OK. I'm wondering if its an impedance mismatch between ipod out and amp in? Seems simple enough but before I go and shotgun the tubes I'd like to see if theres anything else I could look at.

[oc disorder]

ahh must be related on my fathers side..... seriously a couple of things that occurred to me
I presume you are "mono-ing" the signal from left and right with , say 2 x 10k mixing resistors.
Sometimes depending on the design, the two outputs don't mix together well.. i.e. (um) shorted!
I presume Ipods don't have any dc at the output ..a cap between the ipod output and amp input would fix that if it's the case.
Maybe C3 is leaky in fact wouldn't take much to renew those 3 caps C1 .005uF 500v, C3 .0035uF (33) 500v and C2 .047 400v.
After that I guess its fault finding and replacing tubes.

[disorder]

No mixing resistors used when combing left and right signal. Just a stereo male 1/8" to two RCA cable and then a stereo to mono RCA adapter cable.

[g1]

I think what OC was getting at was that the MP3 player might not like having it's outputs shorted together like that. Maybe try just the left or right out rather than combining.
Agree that C1,2,&3 probably top suspects.

[soundguruman]

This isn't a guitar amp but it's a pretty simple 5w hi fi amp. It's a Voice of Music 162 that powers on and passes signal but only sounds good with the volume dimed and the input signal as high as can be. I'm feeding it music from my MP3 player. It sounds great like this but anything below full volume (or if I decrease the volume coming from my ipod) it starts to get very gated. Crackly, gated, choppy

Here's the schematic.

Input to grid of 6AU6 via a potentiometer. All resistors measure OK. I'm wondering if its an impedance mismatch between ipod out and amp in? Seems simple enough but before I go and shotgun the tubes I'd like to see if theres anything else I could look at.

Put a resistor in series with the input grid. It is not unusual to use 1M 5M 10M with a pentode input...