I lived with the PSB Stratus Silvers for more than ten years, and I now own the standard Songtowers. The PSB is a similar ported MTM tower design, though I'm not sure it is technically transmission line. It did have very good bass extension.
The PSBs were mellow, natural and a bit reserved. The Songtowers seem more "up front," but without a trace of harshness or shrillness. I believe this is because the STs are simply more accurate. They image much better than the PSBs--I noticed it in the first 30 seconds of listening.
I don't know if it's the excellent drivers, or the way Dennis Murphy has voiced the Songtowers, but I find them very "real" sounding with loads of detail and very smooth bass. Hope this is helpful for you.
Craig