SVT II Pro Premiere Edition with problems

Hi there!
I just made a nice deal (that's at least what I've thought yet) that got me an Ampeg SVT-II Pro Premiere Edition (071/250).
This thing has been played a lot and it has dings and dongs all over it, but I got it as "technically in perfect condition", with tubes that have "some 20 hrs".
I already played the amp on a gig and it sounded really sweet. That SVT crunch I've missed so badly, I got it back! ;-)
However, when testing it once again at home, I recognised the following 2 problems:
- the mid tone control doesn't work at all (no matter which frequency is dialed in on the 5 position selector). Even turned all the way to the right (maximum boost) or to the left (maximum cut), there is absolutely no change to the sound.
- the bias of the power tubes can't be set the way it's intended (green LEDs on). No. 2 is either red or red and green and No.1 is always red or red and green when turned all the way to the right.
What could cause these problems? I would love to keep this sweet sounding head. but bringing it to a technician who repairs it for 6-700$ isn't an acceptable option for me.
Thanks in advance for your input!

 

Thanks for your answer. The guy I've got the amp from, already offered to take it back. But I love its sound and I'd love to keep it. I'll have to look for a good tech who's able to find the problems to estimate the cost of fixing them. I've found some information on the web that the mid tone control circuit uses one 12AX7 in the preamp circuit. Could a faulty tube cause the non functional tone control?

 

Thanks a lot for your answers. I already contacted a technician, I really want to keep the amp. it sounds so good! I hope it's only a defective pot...