Two Identical Amps - One With Distorted Output

[aniki]

Hey Folks,

After a bit of direction here.

I use two identical amps (Solid State pre amps / Tube Outputs)

Running either one at 8ohms produces identical results.

Running them at 4ohms, one is clear and as expected, the other distorts; even at low volume.

The one that distorts has had the most use but at 8ohms, still runs happily for several hours at a time at a volume loud enough to knock down houses!

Before I start throwing money at valves or transformers, can anyone suggest what might be the issue?

Many thanks

[aniki]

Can confirm that different tubes make no difference whatsoever.

[itu]

FX loop, connectors, cables, dirty pots... if all of these are fine (try different cables), then take the amp to a technician. 

[Bigguy2017]

Do the amps have selectable output impedances (transformer winding taps)?

[Woodinblack]

If they are pure tube outputs with a 8Ω and 4Ω output, and it is ok on one but not the other, then the only thing that can be wrong is the transformer or the socket for the 4Ω connector as there is nothing else, unless there is a problem with the speaker, or the loading is wrong (ie, not a 4Ω speaker), but I assume you tried that with the other one.