While the REC OUT of the AMP1 Iridium Edition simulates an oversized cabinet with V30 inspiration, the REC OUT of the AMP1 Silver Edition and AMP1 Mercury Edition uses a series of analog filters to emulate the frequency response of Thomas's favorite guitar cabinet, a 1970 Marshall 4x12" fitted with old Celestion Greenbacks, each rated at 25 watts and 55 Hz.

BluBOX VSC

The same cabinet can also be found in the form of an IR-based speaker simulation in the BluBOX ("Stack 1970"). To what extent the simulation of the BluBOX differs from that of the REC OUT on the AMP1, you can find out here: What is the difference between the REC OUT of the AMP1 and the BluBOX? ‍

Academy of Tone, Episode 55

In Academy of Tone episode 55 there is more detailed information about REC OUT and BluBOX :