It seems like a self-oscillation to me!
Oscillation may be from the following reasons:
1. RF interfearance(less-likely in tube amps)
2. Heavy speakers with wide swings of impedance up or down arn't a good match to the output transformer of tube amplifier.
3. Cables with high capacitance(be aware that very often high-end speaker cables have worse reactance parameters than a regular cheap speaker wire!). Check if the problem still percist with cheap RadioShack or Home Depot speaker wires
4. Deeper negative feedback helps to reduce or stop self-oscillations.
Oscillation may be from the following reasons:
1. RF interfearance(less-likely in tube amps)
2. Heavy speakers with wide swings of impedance up or down arn't a good match to the output transformer of tube amplifier.
3. Cables with high capacitance(be aware that very often high-end speaker cables have worse reactance parameters than a regular cheap speaker wire!). Check if the problem still percist with cheap RadioShack or Home Depot speaker wires
4. Deeper negative feedback helps to reduce or stop self-oscillations.