Rookie Tube Question s

I have a pair of 6l6g push-pull monos, one of the two power tubes for the left channel does not glow at all. The sound in that channel is still working very well, although it is definitely worse than when the tubes were both glowing. Do I need to replace the tubes? Should I replace both channels (eg two mp) or just the two in the left channel (1 mp). Should I buy a matched quad? I know nothing about tubes as you can probably tell so any info is appreciated?
First thing is to see if it is the tube that is bad or the amp. You can just switch the good one and the bad one in that channel. Be careful to watch it as it warms up. If it glows very brightly, shut the thing off fast! This would probably indicate a bias problem and should go in for service. It can be a simple adjustment or something ugly-you never know.
If the bad tube still doesn't light in the good socket, you have a bad tube. In this case I would personally replace all 4. You can usually just dump the 3 remaining on ebay. If you've never bought new tubes for it, this might actually be a good thing.
Evelick Said it to try to determine if the tube is bad; An easy way is to move tubes around to see if the 'behavior' you are observing follows the tube or the socket, and also keep careful track of which tube is which, I evan write it down on paper as I do this.
As far as the tubes glowing, it is very important to know that the filaments, (the rods and connecters and such within the tube) will glow, but the plates, (the large flat parts, usually painted grey or black) should absolutely not glow at all. If they do, this will damage the tube. It is also an indication of a bias or other adjustment, and this could be because something has failed with the amp, or the tube, or could actually be just a simple adjustment because that particular socket is adjusted for a particular tube. (that is why it is so helpful to keep careful track of which tube came from where).
So- you must first figure out on how to correctly adjust the bias for your amp, make sure you know how and have what you need to do it, know the range or the spec you must set the bias at, and set the bias accordinigly. If you cannot set the bias properly, substitute tubes to see where the problem goes, and if it is not a particular tube, have the amp checked. If it is a tube, get minimum two matched pairs, and preferably, a matched quad. If you are umable to figure out how to set the bias adjustments, you will have to have it checked anyway, and if you get new tubes, you will have to reset the bias also.
And one last thing- get your tubes from a reputable source that actually test and matches them, As there are a lot of venders that sell "matched" that aren't actually matched at all, and you may find that not only will your amp not work as well as it could, but you may not evan be able to correctly adjust the bias for the tubes.
If the tube doesn't glow then it means the heater supply is not working. It could be tube or your heater supply circuit. Try tube replacement first. If same happens after tube swapping then it is very likely you have a broken heater supply circuit somehere. Pin 2 and 7 on the 6L6G are the heater supply pin outs. Back trace them to see if you have broken circuit somewhere.

Hope this helps
By the way, once you get your tube amp working you should try dropping it into your main system. I looked at your two systems and I think you might be surprised at how your inexpensive tube amp sounds compared to your Krell integrated. Particularly with your Verity Audio speakers which I believe go quite well with tubes. Enjoy!