I had a Stebro on my former 164L. I liked it a lot, sounded and looked good, but I didn't buy it myself - my mechanic did, so he dealt with them. I get the impression that they prefer to do business with volume shops rather than individuals.
I have a Magnaflow system bought from Simon here on the AlfaBB on my '74 GTV. I couldn't be happier with it! Looks and sounds great, and was pretty easy to install myself. Only thing different b/w this and Stebro really is tips: Stebro polishes their tips, while the magnaflow system is a plain finish. I took an hour with the tip and a few very fine grades of emery paper and then the buffing wheel and got a nice polish on mine before installing it. I think Simon is running another batch of these soon - check in on the last page of that thread for details (and the first pages for general info on the system):
MAGNAFLOW Exhaust Systems Have Arrived !!!