Tube or Amp Problem? Please help.

Given that I can set the bias for each tube individually is it still important that I replace both on the side that this occurs

Although the bias is independent the circuit is designed to have a pair of matched tubes. 

Next time when you see fireworks (or the sound changes), shut the amp off immediately. That tube is blown. If you have a lot of hours on that set of tubes, it’s an indication that the other tubes may not be that far behind. If that is the case, it’s time for a new set of tubes. Turn down your bias before you install the new tubes. Warm up for a good half hour and then bias the tubes.

On most 2 tubes per channel amps with manual bias, you want to have matching tubes for the pair. They are designed to run that way and it will be hard on other components if they don't match. Some newer, automatic bias amps will let you run any tube but that is not the way most amps are. 

Ref: Upscale has Gold Lions for $92.95 each and Electro Harmonix for $45.95 each, no charge for matched quads.

will it sound bad if I replace the 2 on the side w/ the bad tube with something else? 

Probably not but it might sound different on each side. I saw on the web that those tubes might not be very robust and you have around 1000 hours on them. You can change just a pair but if it was me I'd get a matched quad and start fresh. When you bring the bias to spec and everything adjusts normally, chances are the amp is fine. 

Unfortunately, i don't have unlimited funds.

Not many do but tube amps take more financial commitment than solid state gear. If you want to play you have to pay.