I'm a bit confused by your first post because you refer to the long cable as the attenuating cable but you say it delivers the most heat to the termination?
Yes, that's right.
However, one thing to bear in mind is that your scope + terminator may have >22pF combined capacitance and a 6ft length of 50R transmission line will transform this complex load about 3/4 of the way around a smith chart at 40MHz. So the difference between the short cable and the long cable could be like comparing a 40R load against a 65R load when comparing a 2ft cable against a 6ft cable at 40MHz. i.e. the load presented to the amp will be on a different part of the smith chart in each case. I wouldn't expect this to be enough to cause such a marked difference in level though.
I'll have to Google what you have said
That's the kind of information I was hoping for so that I can dig in further.
Are you driving the amp into saturation?
No, I can increase the output amplitude at least another 50% before I see any evidence of saturation.
How much combined capacitance is there in your terminator and your scope?
I have no idea. I see the same behavior with two different scopes. A Rigol DSO and a '74 vintage Tektronix 475.
Also, are you putting the terminator at the scope end of the coax cable (recommended) or at the amp end of the cable?
Initially at the amp end but I thought better of it and put it and the scope end.
(obviously not recommended as the cable and scope will introduce a complex load that varies a lot with frequency). This would cause all sorts of confusing issues with respect to what you see on the scope and how hot the terminator gets in each case. I assume you aren't doing this but I thought it was worth a mention anyway.
Regarding the terminators, I also used the actual modulators in place of terminators and saw the same behavior.
Right now, I am swapping the "problem" cables with the LF ones I use in other parts of the instrument to see if I can find an acceptable combination for the RF side of things. There are a lot of cables so it's quite confusing and time consuming.