Often this is a broken input connector or an electronic failure such as a tube or semiconductor device.

On some units a failed switch on a jack for direct input to the power amp can cause this. Oxidation at the contacts causes this problem.

Yeah i would consider replacing your eh preamps with some jj ecc83s pramps (its a 12ax7). also, volume loss is usually something shorting to ground, i would assume a smaller electrolytic cap in the preamp somewhere. look for bad solder joints and overheated areas on the circuit board, indicating a potential cold solder joint/ a cracked weld and resolder the part at the least. the parts that get the heavy use are the input jacks volume pots, etc. look there also for bad solder joints.