I posted the same question in the DIY forum, but I realize this is probably a better place !

I am restoring a pair of 4333b and would like to try biamping.

Can I use an ARC EC-21 ?

It has ECC83 tubes.
The low pass is active and the hi pass is passive.
Slopes are either 6db or 18 db/octave.

Has anyone tried 18 db/octave on a 43xx monitor ?

Thank you for your advice.