20 pounder.
There are a couple JB boxes with that nomenclature.
Since you have the one that does 120-150 watts, I'd say you have a low tube... Get that tube replaced, and use IT to drive the Henry.
OR, use the D & A on the LOW side with the RADIO as a driver into the Henry, when the going gets tough.
Hitting a pair of 500Zs with 700+ watts is looking for trouble.... I'm going to GUESS you haven't looked at the price of a pair of 3-500Zs lately... 20 years ago, we retubed the Henry's and Heathkits for 100 dollars FOR BOTH TUBES.
Today, the Amperex is over 300 dollars EACH. The CHEAPEST 500Z you'll find is Chinese, and over 100 dollars each, when purchased as matched sets.
If you REALLY want to hit that amplifier THAT hard, throw about 20-28 volts of bias into it... You NEED that much bias to get the tubes down.
IE, think of it this way, depending on the Henry you have, it's rated for 1200 to 2Kw PEP output with 100 watts of drive.
We will take the 1200 watts, and do the 'maths' from their.
100 watts in = 1200 out = 12 times multiplication.
Your 700 watt driver should push that Henry to 8 thousand, pep. I HIGHLY doubt you'll see that.
I'd hit it with no more than 300 watts PEP, and I'd do a bias mod FIRST.
OR, I'd leave it stock, with 4-7 volts of bias in it, and hit it with the BlackCat. With a JB150 (225-300 PEP depending on the tube), I can hold a thousand watts AVG on the Bird wattmeter... That equates to close to 2800 pep out...... Half the drive, same Pout.
If you have any other questions, please feel free to ask. That BlackCat makes a bitchen driver... For EITHER box... BUT, your IPA (intermediate power amplifier) is quite large for your 'pumphouse'....
And FYI, 20 years ago, 6 500Zs was bigstrappin. Guys are runnin 20,000 tubes now. Step up!!!
--Toll_Free