Hi kes-m0pkz
My Sun2 pro works fine in 4m Rx mode with either of the two the RF (shortwave) input. The internal Sun lowpassfilter does not yet attenuate 70 Mhz band so significantly that it affects the sensitivity.
As 70 Mhz is very close to the LP filter cut off (80 MHz) there may be variations between single units.
For the 4m Tx side, a fast way forward is to look for a comercial 70MHz module like the RA7H0608M or another more powerful module if required.
As we all wait (im)patiently for the ExpertSDR2 1.3 software release, there is enought time to locate a module and get a pcb done.
I HOPE (!!!) the new software would still allow top either use the normal RX or the ADC input and will only route the 4m TX output to the DAC out socket.
In my locations it was absolutely necessary to attenuate strong FM BC signals (6m-4m and in particular 2m) so I had to add a combined bandstop/ lowpassfilter in 4m/6m RX path.
Rx out and DAC input is already occupied with a 2m Highpass filter.
As I use two separate Antennae for 6m and 4m, both with a separate LNA - I had to find (or build) a low loss diplexer.
As pictured, I found a good one one and it saved me quite some time....)
To suppress the very strong FM BC signals, I had already set up a simple FM Bandstop filter (the one with three silverplated coils) followed by a 7th order low passfilter with 75 MHz (3dB) cut off . Such a filter is and was was absolutely necessary for my IC 7300 beforhand!) - it is helpful for the SUN SDR as well.
May be German hams will be allowed to use a part of the 4m band again in 2018 - then we may benefit from the performance of our new toy on 4m as well.
73, Charly, DF5VAE