I have the Westone AM Pro 30x as well and love them ... I've used several others and these are hands down much better. The included silicone eartips work great for me (although I having customs made as well). I'm now using them with an Audio Technical ATW-3255 ... an expensive unit but it provides literally all UHF frequencies legally available in the US. Most others at that price (e.g., Shure PSM300) provide a very limited range of frequencies and you have to purchase a unit with a set bandwidth based on where you think you'll be using the system the most. The AT provides all frequencies, so you're covered (in the US) even when the FCC changes the available bands a little.
I have to disagree with DonPetersen a bit -- the transmitter/receiver hardware matters, but only to the point it can provide a clear channel to broadcast on. I've used cheap and not-cheap rigs, and the difference isn't so much audio tone as susceptibility to interference (both in frequency band to find a clear channel, and RFI/EMI). If you buy the right frequency range in the hardware, finding a clear channel is somewhat binary -- you find one or you don't. Once you do, the quality of the audio doesn't change much from the $100 units to the $1,000 in my real world experience.
The quality and fit of the earbuds, however, matters tremendously and determines 90+% of the quality of the IEM experience, especially the seal of the earbud in the ear. Again, as long as you have a clear channel, the earbuds are the single biggest determiner of quality sound in IEMs.
I love the Westone's because they seal great, but the passive ambience still provides some outside sound and doesn't make them feel so isolating.