John: 

When I tried 15M last week, I got 440W with 60W drive, not sure if that's low for that amp or no. 
20M was 240W with 80, definitely low. As for my FT-102, yes it has 3 6146s in the final and has no issues what so ever on all bands with the antenna. 
The 2ND MFJ-986 also has no issues tuning my antenna while my own tuner still can't handle 500W on the 75M band but can handle 200W no problem at all. 
Before I took possession of the amp, I was told that it came with new tubes installed and a spare set of 4 that wasn't so good. I can't help but wonder if the gentleman put the wrong tubes in for me. When I get the amp back, I'll get my XYL to help me try the spares to see if that makes a difference. 
I'm going to give Zero Five a call to see if in fact those ground mounted verticals induce high currents and voltages on the feed line and if over time with using this antenna, even the Palstar tuner will star to give problems. 
It kind of fits because I had no problems with my antenna tuner until I had a month of use and then the 75M spike started happening. I may even have to consider tower mounting it and having cut to band elevated radials, which I can do as I still got 25 feet of usable tower at my parents house. 
My coax may truly be bad as well, even though I'm making a good contact on each end. This coax came off a 500 foot roll that was installed on a fishing boat that fished in the north atlantic around Greenland and Baffin Island, very harsh climates on coax. 
Nevertheless, it looks like a new set of tubes and anew tuner a fairly big expense if I can't get a refund on my 43 footer. 

73: 
Mike VO1AX