Tremoverb hissing

My tremoverb is pretty noisy, and I'm trying to figure out if it is normal or abnormal noise that I'm hearing. To start, there is no noise when in standby. Once the standby switch is flipped, I have identified two different types of noise.

The first type of noise is a hiss, which can get pretty loud when the amp is turned up. The hiss is present when there is no cord or guitar attached to the amp, and present when the effects loop is on or off. When I place a volume pedal into the effects loop and pull the volume to zero, the hiss goes away. This makes me think that the hiss is somewhere in the preamp, such as a funky preamp tube. I've already changed V1 with a new Mesa Boogie 12AX7, but the rest are old Made in Russia Mesa tubes.

The second type of noise is a low, quiet hum. This is the sound that is present when I drop the effects loop volume to zero as above. Really, it's not even very noticeable, but definitely there.

Anyone have any pointers? I've ready that 12AX7 tubes rarely need replacing, and I'd hate to waste money if the sounds I'm hearing are normal, and not likely to be improved with a tube swap.

It might need some or all of the electrolytic capacitors replaced if there's a hum. Those are the ones in cans (probably blue or black). It could also be a plate resistor failing.

Someone would need to troubleshoot your amp.