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