Something that can work in your car? I've used a small battery powered transmitter that I bought for 15 euro when I originally developed the stereo/RDS encoder; modulation level isn't perfect (it has a few kHz overshoots) but it sounds good enough after (a lot of) calibration, with decent stereo separation etc. But I assume that that's not what you're looking for? If you're testing it in your car, I can tell you from experience (from using an analyzer in my car) that the power supply that you can from the car battery can be very noisy and cause all kinds of problems.
Well, my idea is to mainly use it in the car but I do also want to keep it running inside my house. I am okay-ish with it being noisy in the car. I've been testing it inside my house and it's all over the place while the FM Transmitter has been in the house. I would say that just using that FM Transmitter in the house is pretty annoying to use. I don't know how I would test to check if it's the power supply? Would a bad power supply cause so much tilt + harmonics? How can I tell if it's the transmitter?
In my personal opinion, it would be so cool to be able to broadcast my own signal (even if it's just at home) that isn't shallow, & very loud.
Excuse me for this suggestion if it's not the end result you're looking for since it does not solve the FM transmission issue. To play processed Stereo Tool audio in my car, I use the built-in streaming engine with Icecast and stream it to AIMP on my phone (for some reason the VLC app won't play it). I control the music with Spotify Connect from my phone and have it play on my home PC. And of course, I adjust the sound via the web interface.
Please ignore this if it's not the approach you need.
Well, yeah not necessarily what I really want to do. But that's a good idea. Though I don't really want to use any internet and that will require me to use my data plan.