Are you using tubes on the input....or is it discrete solid state or integrated circuits? Have you tried all kinds (types like toroid or enclosed, wire wound or foil...better wire quality, etc.) of different iron core and air core coils on the output? Everything makes a difference.The earlier prototypes used no input circuit at all since our preamps have no problems driving a 2K load with plenty of voltage. But obviously we needed more gain so we set it up with opamp inputs. Initially we had used an input transformer but the opamps did a better job with wider bandwidth and lower distortion- and were more musical.
We found off the shelf SMPS to really be hard to work with. They usually have current limiting built in which was problematic. So we went with conventional power transformers and that problem was solved- and for less money by quite a long shot than a proper custom-built SMPS would have cost.
The filter chokes do pose their own issues but almost entirely about how well RFI and EMI is suppressed. If that isn't sorted out properly the amp simply won't sound right and no small part because it can affect other equipment if its leaking noise! If I were you I'd be very careful about replacing the filter choke if you don't have access to a good spectrum analyzer with a calibrated antenna as well as equipment to measure the noise the amp might inject on the AC line.
