I've spent the last week or so going through a range of different wireless systems at work, I'm personally going to just go with the cheap and cheerful AKG WMS40 Dual Wireless (with a PT40 bodypack and a GuitarBug) for my 4 and 5 string basses. No loss in low end, even with the 5, the only problem is the single fixed frequency but that's no going to be a problem for me atm.
The Sennheiser is a good unit, for me it wasn't worth the extra cost though as I'd need two of them and the AKG is less than half the price of one of them!!