A little history: Egnater made the original modular tube preamps, then licensed the tech to Randall. Now the same tech is licensed by Synergy and they have made some great improvements, particularly adding a second channel per module. The SYN1 loads a single module at a time but has great versatility in how you can hook it up: straight into the input of your existing amp, XLR (or line) out with cab sim for recording, and hooking it into the series effects loop of your existing amp. This last one is damn cool as it adds two channels to your amp's existing channels. The SYN1 also has a Freyette-designed sag circuit so you can dial in the feel you like. The OS module is a Dumble Overdrive Special inspired preamp. It sounds wonderful into my recording rig via the XLR but where it really shines is in the effects loop of my BassBreaker 15. Using BYPASS of the included footswitch I get all of my BB15, then switching the SYN1 to on bypasses the BB15 preamp and gives me the clean and overdrive of the OS module through the power section of the BB15. Super cool. Another config that kills is putting the SYN1 into the effects return of my HRD. This way I get the two SYN1 channels through the louder power section of the HRD. The system is not cheap ($500 for the SYN1 and $400 per module). I may invest in the Friedman BB/BE module at some point but for now I'm pretty happy with the OS module!