The most significant difference is the pre-amp. The pres in the two units are similar in performance but not identical. The original Trak Master has a typical class A Platinum design, (based around a 737 Rhom transistor) but the TrakMaster Pro uses an op-amp design closer to that in the Focusrite Green range. Both designs are solid-state, have an extended bandwidth (e.g. up to at least 180kHz at the top end) and are sonically transparent, but there is a subtle character difference; the original Trak Master has the edge in being more forgiving if you want to slam the pre hard. More 2nd order harmonic distortion is generated at high input levels giving the pre a warmer sound when run hot. The TrakMaster Pro op-amp runs in a class A mode (though it is not true class A) and is ultra-clean sounding but if you run it hot it will not sound quite as warm and forgiving as the original Trak Master. Hence if ultra-clean appeals to you then the TrakMaster Pro has the edge, but if you like to drive a lot of level and are not obsessed with total linearity the transistor-based Trak Master, VoiceMaster Pro or TwinTrak Pro is a better choice.
Other feature differences are:
TrakMaster Pro features a Lo-Z (low impedance) setting for use with older ribbon mics.
Trak Master features a 3 band post-compressor EQ (although it can be switched to pre-comp) whereas the TrakMaster Pro features a pre-comp mid-scoop EQ control only.
Trak Master compressor features tube sound and release controls, not found on the TrakMaster Pro.
TrakMaster Pro features a port-hole style moving coil peak level meter which can be switched between input, output and comp (gain reduction).
TrakMaster Pro features a latency-free monitoring section that allows you to route the output of your DAW through it and create a monitor mix with the signal coming directly from the unit (headphones and speakers).
If you are evaluating the two units to decide between which one to purchase then you should consider what you need the unit for. The original Trak Master will perform equally well in live or studio situations - the EQ is great for getting a good sound quickly which is essential for live work. The TrakMaster Pro is more aimed at computer recording setups where people tend not to EQ when recording unless it is to remove a resonance or noise - this is why it has a mid-scoop EQ only. The compressor on the TrakMaster Pro is designed to be ideal for tracking (fast attack and release) and quick to set. Also, the latency-free monitoring it offers is essential when working with a computer.