AT-100 features and specifications
â€¢ All-Tube Signal Path
â€¢ Tri-Mode Rectifier: Tube - Class A or AB,
â€¢ Solid-State - Class AB
â€¢ Overdrive Channel: Gain 1 and Gain 2
â€¢ Each with Independent Master Volumes,
â€¢ 3-Band EQ, Assignable Contour Control
â€¢ Clean Channel: Master Volume, 3-Band
â€¢ EQ and Bright Switch
â€¢ Global Controls: Volume, Low-Res
â€¢ (Resonance) and Reverb
â€¢ Effects Loop with Level Controls
â€¢ Line Out with Level Control
â€¢ Selectable 4 / 8 / 16 Ohms External
â€¢ Speaker Jacks
â€¢ 4-Position Footswitch: Channel, Gain,
â€¢ Reverb and Effects Loop
â€¢ Ground Lift Switch
â€¢ Dimensions: 27" x 11" x 11"
â€¢ Weight: 53lbs
The AT-100 features rock-solid 13-ply Baltic birch construction
AT-100 Tube Chart
Power: 4 â€“ 6L6 / 5881
Preamp: 4 â€“ 12AX7
Phase-Inverter: 1 â€“ 12AX7
Reverb: 1 â€“ 12AX7
Effects-Loop: 1 â€“ 12AX7
Rectifier: 1 â€“ 5AR4
Not a power tube issue since you have 4 and could probably run the amp on a specific choice of two only, but that's not it.
Maybe the biasing circuit went, but maybe something in the speaker cab went on the fritz! Try another, or blame the amp?
So, use another amp with your speaker/cab, or try the amp itself with a different speaker or cab & look inside cab too?
I see from specs that you can do without a reverb tube for awhile, and swap it to phase inverter or the "push-pull" function circuit, see if it works. I only presume that the amp could work with no effects loop tube too, if you damaged it, switch off.
You're new, but you seem to have been asking around. Anyway, I can't speak for the forum, just my own advice and what I would earnestly, or honestly do myself. I'm glad you can use the Digital Multi Meter, but don't touch anything hot or "live"!!!