Playing around with the Airspy HF+ SDR setup tonight since I am in the den away from my usual radios.
I already had a KD9SV SV-CMC choke just upstream of my bias tee, plus some snap-on chokes at the mast base of the active minwhip antenna. I just added six turns of RG-316 on a Fair-Rite likely mix 31 or 43 snap-on choke between the bias tee and SDR. I did not measure actual numbers, as the noise floor was already decent IMO, but the waterfall looks a little cleaner in quick skim.
I suspect it was slight noise from the bias tee, as it is powered currently by a wall-wart. The wall-wart is a linear transformer, but it probably could use extra filtering. I have bias tees to which I added filtering caps, but I think this one is still stock.
I might change the bias tee for a filtered one and go back to a regulated linear PSU later tonight. I was trying to clean up some of the clutter by ditching the bigger PSU, but I suppose "oh well."
Went ahead with the components swap to a bias tee with filter caps and a regulated linear 13.8v PSU. Probably not much difference, at least regarding the HF noise floor, but ideally it should be the technically "better" configuration. Anyway.