I've built a new version of the amp which keeps the linear regulator for lower LF supply impedance, but instead of the LM338K I've split this into two smaller package devices (LM350T). The reason for this is that being TO220 packages they can be placed right next to the amp chip on the same heatsink. Closer proximity means lower resistance connections.
Another modification has been to make a slight reduction in the supply voltage to the chip, for the purpose of being able to use lower voltage capacitors (16V) - these take up significantly less space than the 25V ones I was using previously. Meaning I can get more uF close in to the chip. Running the chip at 15.5V now means I can run comfortably from a 20V brick supply with enough headroom for the regulator. The brick supply, being a SMPSU, needs fairly aggressive filtering and this comes in three stages - two of normal mode and one of common-mode. For normal (differential) mode, HF filtering is done with ferrite beads and clamps, together with ceramic caps then there's audio band filtering with metal powder toroidal chokes, with electrolytics as the shunt elements. The common-mode filter has been moved to the mains side (so as not to compromise the output impedance of the brick) - I'm using the same segmented former but with thinner wire. Getting the filtering good enough has been rewarded with an expansive soundstage, something I've never achieved with any previous chipamp design using the usual suspects.