I recently bought a JTS Guitar system. Good quality UHF Diversity system for £200 in UK. The belt pack is the toughest I've come across in the price bracket. JTS > Products
I use an AKG WMS 40 unit with a PT80 transmitter, and I have noticed no tonal colouration at all. I used it in my bass playing days as well, and it has continued to serve me well for years now. I believe they have updated the models and the new one may not be the WMS series now, but for the money it's been a great set up.
I've had mine for like 2 years now. Works great. I have the guitar one and it works on both guitar and bass. It has a really far range too. I was at a friends house for band practice, and we had to meet my mom down the road to pick up a mic. I decided to bring the bass along. Walked all the way to the end of the road (a good football field, field and a half) and it still sent the signal and got to my amp.