It's been almost 2 years since I played with the guts in my Kalamazoo so I am working from memory  First I would put some cathode bypass caps in the preamp next take out the linking resister between the volume pot and the next stage (just sub in a peice of wire)I did go back in and change some of the preamp resisters to get more gain But the biggest improvment was the added caps Try some 25 or 30 @ 50V . You can also add a bypass cap to the output section since this amp is cathode biased I would try a 50 mfd in there first and see how that works for you 
I ended up replacing the Output tranny (I used a fender deluxe) the output bias can be run a bit hotter with the bigger transformer But I can't remember what I values I use
I also ended up using a Celestion vintage 10 that helped a lot since it is a lot more efficiant than the stock speaker.

I bypassed the el84 cathode resistor with a electro cap for more bass and gain. For harp I would use lower values (4.7uf) and move up until it sounded its best.