Is this bad?

My modified Ming Da MC34-b has outputs for 4 ohm and 8 ohm speakers. Well my speakers are rated at 6.5 ohms. Is it proper to use them with the 8 ohm jacks?

The reason I ask, last night I noticed noise for the first time, then it got louder. I noticed one of the output tubes was glowing much brighter than the others.

Maybe this is a bad tube instead?

Sorry for the lack of specifics, I'm new here...
Select the output taps based on sonic's. Electrically it won't make much difference unless your speakers have a significant minumum impedence, in which case the 4ohm taps will probably sound best (control the bass).

The tube issue you mention is totally independent of the tap issue. You have either a bad tube or a biasing issue I suspect. It could be either. Are these new tubes or have you been using them for a long time and one is just now acting up? Have you checked the bias on these tubes lately?
The amplifier was a recent AudiogoN classifieds purchase. The seller indicated that the bias was set just prior to packaging. The tubes are fairly new.
Do you know how to bias the tubes? If not you should find out and check the bias on the tube. Perhaps the distributor can give you this information if your don't have a manual.

You could have a potential major tube failure in your future which might damage some amp's resistors. Unfortunately I don't think just dropping in new tubes without re-baising is a thing you want to do either. I wouldn't recommend that you continue to use the amp until your solve this issue.
I'm thinking I'll rotate a tube from a different socket, and test it out just long enough to see if it follows the tube.

Apparently the amp needs to be biased by a technician. I could handle it with proper instructions.

Thanks for the info!
Here is a possibility, assuming that the guy you bought this from was honest about biasing before sale. I'm sure you thought about it...If he did it, he knows how, can tell you how, can tell you the optimum bias point, you can get a meter and check it out. Not to be grim, but I wouldn't be surprised if he doesn't have this info. Quite a few Chinese amps have made it to our shores w/out manuals, bias instructions, etc, making maintenance difficult for non techies. Good luck.
he had it modified for different tubes, replaced the caps and resistors, and updated the wiring. Aparently he had his tech that did the mods bias the amp prior to sale. So at worst I can get the info from him. The bias adjustments are supposedly not "user accessible" but I think I can handle it with clear enough instructions.
Was the amp shipped with the tubes in place? If not were they labelled? If the answer to both is no, then whether or not is was recently biased is irrelevant, it will need to be re-biased.
Most likely your amp has bias adjustments for each output tube. Each output tube needs to be in the same socket that it was in when the tech performed the bias adjustment or you have to start all over with bias adjustments.

If the seller took the time to mark each tube for the proper socket before he shipped it, you might be able to move them around to their proper location and be up and running again. If not, and if the tubes were simply shipped independent and without any indication of which socket they go in then the seller didn't take his responsibility of selling serious.

Tubes can vary in how bright they are and still be biased properly. A tube with a filament that extends higher above the plate structure than another will glow brighter. However, if the plates are the least bit orange or red then you have a problem.
The tubes were removed, well labeled and shipped in a separate box. The amp was labeled as far as what tube went where.

I've been listening to it now for 3 hours straight, and the problem has not recurred.
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No, the tubes were not labeled per socket. So the output tubes could be in any of 4 slots, and any of 2 for the input.

I'll contact the seller and get biasing instructions.

I've not had another episode of noise like the other night. I listened for 4 hours last night and it worked fine. So for now I'll make sure I'm around while I listen, and get it biased as soon as I have the instructions.

Thanks to everyone for your help.