ARC D-115: What tubes and cables?

Hello all, I am seeking input on which tubes and cables will help in providing neutrality and extension in the highs for this amp. The tube complement is SED 6550 with 50% life in them and brand new Gold Lion and Sylvania NOS for the driver tubes. My dealer suggests KT88s slhould be next.
Any input would be appreciated!
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No KT88s for D-115. SED 6550s are good, but must be matched. I liked RCA clear top 6FQ7s. Tubes from Audio Research will give you the most neutral sound, but they will cost you plenty.
Agree with Rrog SED's, Winged C's, 6550WE or Philips 6550. The amp was voiced with the Philips. Prefer the GE 6FQ7's for stock. RCA Black Plate 6FQ7/ clear top for nos. If this is a D-115 plain and not the MKll, 7308's instead of 6922's for drivers.
You can't find real new old stock 6550s anymore. There may be a precious few here or there but the cost will be comensurately high. I don't know why you guys don't like the KT-88. It is a very good tube and the current production Gold Lions produce beautiful music in the systems I have used them in.
If you insist on 6550 output tubes they will also have to be Russian. I don't care for the 6550 sound so I can't suggest one over another. I think I have an amp that has EH6550s in it. It is in storage so I can't say for sure. I didn't like them that much I remember.
The drivers are another story good NOS should be "findable". Has an yone thought about using the KT-120s in an older ARC?
Thanks guys, this is the first tube amp that I've owned.
Prior to this, I was using an ARC D-120 that simply got out
of the way and made my Proacs really sing.
However this is an entirely different beast! (LOL)
I always used 6550 tubes from ARC with this amp.
In your header you also ask about cables.
I found Purist Audio to work extremely well
Goldeneraguy, I was using Radio Shack 18ga solid core cabling with the D-120. (No kidding). So Purist seems like a gigantic leap. Any other sugestions?
I'm using SED 6550's which are a great tube in my D70Mk2. Also using a pair of Nordost Red Dawn 2's...very quick and open/transparent cable. If you can source a set of GE 6550's, they would also be a great match. BTW, KT120's will NOT work in the D series amps.
Wahwah ,Yes Purist wires would be a giant leap ahead.
Magnan worked well at a more reasonable cost
Without knowing your complete system its difficult to suggest what should work for you but I would upgrade the Radio Shack wires before replacing the tubes
Yes!-Sorry for not being specific on the issue- The RS wires was one of those things that simply clicked with my system, and I was really surprised by this. And NO,they do not work well with the D-115. (It is a Mk 2 Hifi Geek).
Here is my System:
Modded Sony DVP 7000
Berning TF-10
Audio Research D-115 Mk2
Proac Studio 200
Audio Magic Xcallibur interconnects/cables.

One interesting development: Last night in frustration I angled the speakers INWARDS quite a bit from the usual position and Imaging improved nicely! However, they still do not dissapear as when I was using the D-120 with the Rat Shack solid cores.
Davef, Thanks for the Nordost recommendation!