Ok, I've spent some time on this yesterday and I have decided on what I want to do.
I'm going to get 3 boards populated, using the PE43711. I'm going to have everything go out to headers such that I can use an external control board to control them.
Because I don't trust myself not to screw up I'm going to include DC blocking capacitors on each of the boards. However, since for going down to 9 kHz you need very big capacitors (in terms of RF-capable capacitors at least) I think I'm going to have different versions with different value capacitors. I'll have a version with some bigger capacitors for going down to 9 kHz, and have the other two use proper RF capacitors that have much higher self-resonance frequencies.
My current idea is to have them all attached to a 19" rack. I'm thinking of getting small metal shield boxes to put each in separately and mount that to the back of the plate, but I will have to see what is possible here.
I'll also have to see what I do regarding control system. My arb is floating, so it might be interesting to keep the modules floating as well and use some form of optoisolation to control the thing. Then again, if I keep them floating that means I have to make sure that the connectors and shields are isolated too, so it might be more hassle than it is worth.
Control will be simple - just an arduino and an LCD. Perhaps three sets of up-down buttons (+-0.25 dB, +-3 dB and +- 10 dB?) and a "channel select" option? I'm gonna try to get the RF boards out as fast as possible, so I have them in a few weeks for testing.
Will keep you guys updated and am always interested in feedback.