Little late, but here's a mod/rebuild service for the PL-400.
Not familiar with them myself.
https://www.worldproaudio.com/WPA-Phase-Linear-400-mod1
I used a 400 to power stacked Advents in the mid 70's and it worked/sounded fine.
However, I tried it powering Altec VOTT's (24+ hour party) around the same time period and it would shut down after an hour, so.
We ended up using one of my Dynaco ST70's for the rest of the party without a hitch.
The PL worked fine the next week powering a pair of large Infinity speakers that the ST70's would not properly power.
The preamp used with both amps was a Dynaco PAS3X, which may have caused the PL problem (though it was the same preamp used with the double Advents.
Go figure.
DeKay
Not familiar with them myself.
https://www.worldproaudio.com/WPA-Phase-Linear-400-mod1
I used a 400 to power stacked Advents in the mid 70's and it worked/sounded fine.
However, I tried it powering Altec VOTT's (24+ hour party) around the same time period and it would shut down after an hour, so.
We ended up using one of my Dynaco ST70's for the rest of the party without a hitch.
The PL worked fine the next week powering a pair of large Infinity speakers that the ST70's would not properly power.
The preamp used with both amps was a Dynaco PAS3X, which may have caused the PL problem (though it was the same preamp used with the double Advents.
Go figure.
DeKay