Was the previous supply a simple transformer, rectifier, and filter?
I assume the converter's operating frequency is well above audio frequencies and that it is filtered. That in itself should not present any problems. You can run PMDC motors at 16KHz without filtering as the motor's inductance will keep the current looking like it's DC. We've had problems keeping switching devices cool above about 30KHz. There isn't much advantage going higher for a PMDC motor load and all the extra aluminum required to keep the switching devices cool gets expensive. Inverter drives generally impact the integrity of the insulation system and bearing life more than brush life.
How stable is the power supply's output when driving a highly reactive load? PMDC motors are highly reactive on startup and having current lag voltage can cause feedback problems within the supply, especially during startup and also after speed changes. Within reasonable limits, brush life is typically a function of the inverse amperage squared so big current spikes will eat brushes.
Is the output being ramped up or just switched on? How many commutator segments are there and what is the RPM? Is there a flywheel diode across the motor?