ARC D-115 ..need advice

My D-115 has been sitting for a while because I'm not sure what do next. It originally came with the great Phillips 6550's. They lasted about 5 years. Bought a used set of GE 6550's from ARC that lasted about the same amount of time. Sounded almost as good as the originals.

The Phillips tubes are shot and can't be reused. Most of the GE's test out fine on my old Hickock tube tester.

Is it possible to find GE's? What would the cost be? Since there are 8 power tubes, would it be possible to match the set by buying just one or two tubes?

What if I decide or have to buy some of the newer versions available? Any recommendations? Most folks on this forum don't say much good about Sovteks, preferring the Tesla or Svetlana tubes. I remember the original reviews of the Sovteks were favorable, however. Who are the best and most reasonable in terms of price of dealers to contact? KT-88's or 6550's?

Sometimes, when the tubes go, they also take out a resistor in the same channel. If I need to go to a dealer who repairs ARC products, do they generally let you bring in your tubes when rebiaising and repairing or do they insist on staying with the high priced ARC stuff that they order? Who are good dealers to work with in the Midwest?

One final question, which tubes have better reliability and longevity?

Any assistance would be most appreciated. I'm currently using an acceptable Dyna ST 70 that was modified by John Hillig. Although it's a good little amp, it really belongs in a second system, compared to the great sound put out by the D-115.

Sorry for all the questions. I want to make the next step the right one, so my amp doesn't have to sit on the shelf too much longer. Yet, I don't want to spend an exorbitant amount of money.
I also have a D115. I have tried both 6550 and KT-88. I am using KT-88 for the sweeter sound. Most of the so call matched tubes you paid premier price for from dealers are not really matched. If you have 1 out of 8 6550 that's shot, don't replace them all, just get another one and adjust your amp a little bit and it will do the job. I have substitude different brand but the same type and it works find. After even a week, even perfectly matched tubes will not be matched ferfectly.
I owned the D115 MKII for 11 years, before replacing it with the ARC VT100 MKII. The D115 mkII was a special and very musical amp. I had great results with the Sevetlana 6550C.
Upscale Audio sells the matched Octet for approximately $240. Their tube matching is excellent and is highly recommended.

Good luck and happy listening with your amp.
The Svetlana 6550C's are very good and appear to be very stable as well. I have a set in my ARC D-115 Mk2 which I am shamelessly going to say is for sale, great amp btw, just don't need the power and it ISN'T as good as the OTL that replaced it. Have about 1K hours and the tubes are still testing very strong. ARC uses Svetlana for replacement tubes these days as well as in their new amps. The Sovtek output tubes I'm less familiar with but if they are anything like their smaller triode tubes I wouldn't be too impressed. Try this link for 2 matched quads which will set you back about 200.00 for the 8 tubes. Triode Electronics, they have VERY good prices on their tubes and have just about anything you are looking for including Sovteks which are a bit cheaper than the Svetlana's.
One more thing, so far as biasing it is indeed a pain with this amp as the contact points for testing are between the tubes and there are high voltages to work around but if you can find a dealer who will let you watch as he sets the bias with the right tool you can do it yourself. You will need an alligator clip or hemostats to hold the probe in place (better yet a longer probe) and keep your hands far enough away from the tubes as you adjust the bias to an ideal 65ma with a digital voltmeter. It isn't as difficult as it may seem but ARC sure could have made it simpler.

If you are planning on keeping the amp you really need to do it yourself and save some money. Any electronic shop should be able to do it and the schematic clearly shows the points and adjustment screw for each of the outputs.
Many thanks to all of you for your valuable input.