They are similar, but sound different. The SL+ was one of the earlier single channel modules. A bit woofier and a bit nastier on the top than the smoother SL2. I had one which I ended up having converted to a HBE by Friedman. SL+ is a good Marshall sound though.
Totally different marshall vibe in the HBE. Sounds more like a modded vintage Marshall than the SL+ which sounds more along the 2203, JCM800 voicing--fatter sounding. HBE is smooth but more emphasis on the upper mids and a very tight low end.