Can you try bypassing the crossover, taking the speakers wires straight to a jack socket. Try the cab like this and if it works OK then it is the crossover and a repair or replacement is needed. You may however like the sound of the cab without the crossover in which case just make the bodge permanent.
Also just covering belt & b*lls. The bass doesn't sit flat though as large tum gets in the way but I've got used to it. Yes, the large tum and my bass not sitting flat.
PP3 across the speaker terminals of each speaker is the first thing to do to check that the speakers are OK. Disconnect the wires first. If the speaker gives a nice thunk then it's OK. No sound then no speaker.
Set the multimeter to ohms. One probe on the earth pin on the plug and the other probe to a bare bit of the amp chassis or jack socket nut and you should get a reading this means that the earth is good. No reading and the earth is bad.
One of these, http://www.vintagehofner.co.uk/selmer/speakers/spk15.html. A match made in heaven.
I used a a pair of these with a metal clad T & B 100 in 1968, sounded wonderful. Mine just had Goodmans 18s in them though.
I just play it until I remember it. Then learn the lyrics on top of that. I find that learning both helps a lot when it comes to arrangements. Once I've played it live once, it normally stays there and becomes second nature. The problems come when I have to think about it then I forget the lot.