Anyone using KT88's in D series ARC gear?

It has been a long time since I owned my D70. ARC amps of that period have a limited range on their bias pots. So the answer is without modification, maybe. Turn the bias pots down and install the KT88s. See if you can turn the trim pots to the proper bias. I never tried KT88s in my D70. I did run them in my Classic 60 and a slight modifications needed to be made so that they could be biased.

The Speaker strips were always a problem. Though I think spades have gotten a little bigger over time. I used Audio Research speaker cables with my D70 so I had no problem. Excellent cables for the money with ARC equipment.

How are the 2 large Capacitors? If they have not been replaced they should be checked. They may need to be replaced soon.

I agree with Rrog, The amp is a classic and I wouldn't change anything on it. I would have the caps checked because they can leak when they get old. You said that it is currently down. Instead of spending money to modify it. I would consider sending it to Audio Research to have it fixed. They will go over everything and bring it back to the original specs. Put a set of new tubes reputable dealer in it first. That way ARC will not have to replace tubes. I sent my V70 in a while back and I was amazed at how good it sounded when it came back.

To replace the captive power cord with a IEC socket might be difficult. There in probably not enough clearance between the case and the board.

There are some speaker cables with smaller spades or you can replace the spades with smaller ones. As I mentioned I used and still do use Audio Research speaker cables. They are really nice cables and can be picked up used for not a lot of money. The spades will fit between the dividers on the strip.