Fixing my Fender Hot Rod?

Hi, My Fender Hot Rod amp is down and there is absolutely no possibility of getting any help to repair this because of where I stay. I have to do it myself even though I don't like it.

I am getting a very prominent low frequency harmonic disturbance (around 30 - 40 Hz, by the recorded waveform) when a play loud chords. The problem started when I played the amp for quite long and loud one day and it went out - the roaring tiger becomes a mewing cat. I swapped the tubes after waiting many months for them to arrive and it fixed only part of the problem. The volume went up a bit but not much. Now I am left with this zarring distortion problem too. The amps worth a lot of money for me but unusable.

Question 1> Can changing the speaker help?
Question 2> Will a "cap job" help?
Question 3> I cranked up the bias and it made absolutely no audible difference. Probing a bit more, I noticed a significantly lower voltage where it should have been 200-300 V across one of the tube pins. Is that the likely problem? If yes, how can it be fixed?

There are plenty of transistor based electronics repairmen around here who can probably do a cap job for me or even repair broken speakers but they know nothing about tubes and are universally tone deaf. I took the amp to some of them and they all thought it was working fine, just because something could be heard on the output!! PLEASE HELP ME!!