Judasmac, it looks like another tubeophile is beginning to emerge.
Tubes can make a profound difference in an amplifier. In order of priority, I would say either circuit design or transformers are the most important aspect of a tube amplifier, depending on the day, followed by tubes themselves. Except in very rare cases, one does not do anything about the circuit or the transformers, so that much is set. Where most of play around with tube amps is tube rolling. It's analogous to maybe a meal. If one orders a cheeseburger in a restaurant, you might not be able to do much about the patty or bun. However, you can paint a wide spectrum of tastes depending on the cheese, sauce, or fixings. Cheddar, lettuce, tomato, bacon, and mayo sure tastes different than tomato sauce with mozzarella.
How do tubes sound different? Some are punchy, some are soft, some are more neutral sounding, and some may be sunny and warm. If you are looking at a Jolida with the stock tubes, you will realize some very real gains by moving towards European tubes. While I have few complaints regarding a stock Jolida used in the right system, they really seem to open up with a tube upgrade. Again, I think the sound definitely takes a definite step up with either JJ or Svetlana EL34, which you will prefer is dependent upon you. And, don't forget the small signal tubes, as they are very, very important. I prefer to get the right output tubes first, find where your tastes lie, and then take it to the next level with the small tubes.
Incidentally, don't neglect a power cord upgrade, either. In no way do I recommend buying a power cord that costs more than the Jolida itself, but a nice homemade power cord, or something good you can find under $150, perhaps a $50 Absolute PC, is definitely the way to get as much out of the amp as possible.
I believe you may not be able to use 6550, KT88, or KT90 tubes in a 302b without modification. I may be wrong, and in fact, hope I am. The amp that uses these larger tubes is the 502b, and I think may have some different components, even though they are same component otherwise. Incidentally, I do not recommend the 6550 based 502, its sonics are much inferior to the EL34 based 202/302 products. I firmly believe the extra money and power the larger tubes offered in the 502 is more than mitigated by its poorer sound. From recollection, the 801 seemed to correct most of what the 502 took away, but some will still always prefer the magic of the EL34.
