Better speakers is all I want.
I don't have an SDS to compare, but the sound of my 536 blows compared to my 996P2 & T.
I still love the sound from my old Electra Bearcat III and other Electra crystal models from that era as well as the newer Electra programmable models like the BC220, BC250 and so on. All before Uniden bought out Electra or Masco Corp.
They all had much better audio amps in them compared to most of today's models. At least those that Uniden sells anyway.
The worst audio was the original BC100 programmable handheld regardless if it was still made by Electra or the same model after Uniden bought Electra. I could barely hear that portable if there was any ambient noise around me.
The BC100 was unique though being the first programmable portable on the market.
As far as an unknown HP-3, I'd look at one as long as it had a true front end that is made to handle LSM otherwise I probably would pass on one.