Yamaha DTXpress/DTXplorer/DTXtreme

Flynn,

As far as I know, all Yamaha pads are compatible with the DTXpress. 
The DTXpress II has settings for the RHH130 even though it wasn't
available at the time, and is part of the DTXpress III Special kit
anyway.  The TP100/TP120 will work fine, but the red knob will not do
anything (it can control the snare effect on the DTXtreme).  You will
get all three zones.  The volume control on the TP100/TP120 will work.
 All the Yamaha cymbals will work.

Other manufacturers pads may cause problems of too much or too little
signal, although there are plenty of things to play with to try to
sort that out - the attenuator switch, trigger type and gains.

Someone recently posted here about good results with a Roland Hi-Hat I
think - try searching the archives.

I have tried a Roland mesh pad (single zone tom pad) on my DTXpress
and it worked fine (only a brief test on one I borrowed).  The Pintech
Concertcast works fine as well, but the dual zone one uses two piezos
so either needs two inputs or an adaptor (see "magic box" or "DTXpander").

In general, all pads use piezos.  The differences are how much signal
you get and also how well damped they are.  A pad which "rings"
considerably may give double triggering which would need some tweeking
of parameters to sort out (e.g. the self rejection).  Also, some
manufacturers pads use two piezos for dual zone rather than one piezo
and FSR.  They require two inputs or the aforementioned magic box. 
The polarity could be important - the Yamaha modules seem to like a
positive pulse, although with most pads you get some ringing so get
both anyway.  A well damped negative pulse may not work very well, but
it is not something I have explored in any depth.

Keith.