I just use the self powered studio monitors in my studio. But if you need a portable then any PA or full range keyboard amp would work.
If you want effects or strange sounds, then just use the computer (or an ipad) and plug ins. The options are unlimited.
If you really want a module, there are tons on ebay. Romplers, synths, etc. As for romplers, you can get a older roland sound canvas for $60 all the way up to a yamaha motif xs, which might be like top of the line for $1K. Or you can get a killer desktop synth module like a Virus, or a new roland.
FWIW, I use a Korg M3 desktop module and my computer for soft synths. The M3 has this strange karma mode that I am in love with.
I have a malletkat w kurzweil sounds. The sounds are great. There are some tweaks to make the sound sets malletkat friendly and the more useful mallet sounds are clustered together. Not that you couldn't tweak that with any module.... But using external modules or samples is so easy - hard to say the board is worth 700+ shipping.
I would hesitate to spend 700 for the upgrade just for the module. I'm only a beginner mallet player. If I was a hard core player and gigged with it, then the upgrade might make more sense - or buy another mallet kat as a backup and get with the latest options. Or maybe do a trade in? Alternate Mode is super friendly as I'm sure you know. The newer malletkats are much lighter...
Are you sure you can't install the upgrade yourself?