Before I give you my feedback, I'd like for you to know that my advice is based purely on my own experience and observation alone with my BM. My experience might be the result of unlearned tweaking of my BM or takings of a wrong approach to setting it up.

In a short, I would go for a OTS or HRM but probably the OTS if you're looking for that typical dumble sound. I'm not saying that that those sounds can't be had out of a BM, but boy it's hard work. I've had my amp for about 2 months now, and I'm still looking for those tones out of my amp. I've changed tubes, speakers had additional mods added to the amp, Clator built in and have extensively tweaked the amp on both the external controls and internal trim pots. The result has been great now, and the OD is sounding great and is certainly getting me closer to those dumble tones I was originally wanting. I do plan on adding an EQ pedal into the loop in which another BM owner on the forum has had some success with, and I should find out today if this gets me sonically there when I pick up the pedal later today.

As you can see, the journey has been a big one, but in saying that, the tones I'm getting out of the amp is nothing short of amazing. The cleans on the BM are immaculate and is by far the best that I've heard out of an amp, but that those tones were easy to setup. The OD on the other hand has been where the journey been. The OD today is to me definitely Marshall flavored now and sounds great after adjusting the internal hrm trims, and this is where the OD has improved the most. Previously, the OD was either too little or too much where it began to get fizzy, but now it I'm able to get some very nice tones out of the channel with PAB engaged. As a matter of fact, these tones are excellent on their own right.

So in saying that, the BM is amazing in its own right and I think the dumble tones can be had out of it, but it's hard work, well for me anyways. The OTS I think, will be easier to get those tones you're after. The BM is probably a better amp or those looking for their own tones, where as the OTS is probably a good amp for those looking for that typical dumble tones.

Absooooolutely!  Pleeeeeeeexi!  even with 6l6's.  You oughta hear the same 50wHRM with VOS platinum matched RFT el34's though.  Unreal preamp tones with similar Euro tubes made Plexi era used in them from RFT, Amperex, Siemens, Tungsram, Military mazda or Mullard depending on guitar and cab used.  External c-lator uses 50's Siemens longplate, a wonderful tube for that position.

Had both of my D-clones opened up to the guts sticking up with all paws to the sky in submission during a four hour long session of trimmer tweaking and tube rolling.  Started messing around with my 50w HRM LNFB mod chassis first.  Traced back the shielded wires from the OD trim and they came to a pot.  In the past I noticed that for a similar amount of gain as my OTS I had to crank up the trimmer higher on this HRM.  I noticed that the entrance trimmer was less than halfway up.   Fired up the amp and let warm up a good half hour.  Strummed strings, walked over to the amp, had a hand behind my back and set the trimmer nearly wide open.  Back panel trimmer much more responsive now.   Went to the HRM Stack, dialed in a bit more bass and treble.  I adjusted them while leaving the mids untouched.  A small amount of trimmer movement went a long way there.  Huge flexibilty in that HRM thangie, I'm glad for it and how the amp sounds overall.  Went to PI trimmer and very carefully noted original position, adjusted via ear with high master volume setting, new position, only slightly different from original, but noticeably better tone.  When it's dead on I also noticed more presence.   Tube rolled the preamp for V1 and V2 for a good while.  Settled on early 60's Raytheon Long Blackplate for V1 and Philips Canada (Canadian and higher quality version of Bugle Boy 12ax7 in V2.  American tube in V1 lends Fendery cleans, Eurotrash design in V2 wonderful for the OD voices following that particular V1 in this particular circuit.

Went to the OTS, LNFB mod, pulled the 6l6gc Sylvanias and replaced them with a pair of closely matched and adapted pair of 60-s/70's vintage 6bg6ga GE blackplates, warmed them up, set the PI trimmer, again only slightly different from original, rolled more pre tubes.  That amp will rip your face off with some very agressive tones the way the circuit is set up when the LNFB is lifted, effect is dramatic, not subtle as done in my HRM.  Martshall tones easy to get even with 6l6's or especially these unusual, low cost, amazing sounding secret weapon 6l6 variant tubes.  flip the switch the other way and you get very, very good typical ODS/OTS tones.  Rolled some more 12ax7's and then rechecked the PI setting on the HRM. I had left it running the whole time.  That amp wanted to be a SSS on the clean channel, it could sustain cleans amazingly well and get some nice bloom.

Actually I've got the LNFB on my BM, but to be honest I haven't used it yet. Guess because it's concealed at the back where the preamp tubes are. Guess its out of site, out of mind! Will try when it out later tonight!

Holy crap!! really? Depending on were it's set at right now, it may make a rather large difference how you approach the amp, especially on the OD channel. Get mess'n with it!!  

If I had to guess, it was probably shipped with it in the lifted position, like a regular BM, but you may find the OD more pleasing, or atleast more tame-able with LNFB. Either way it changes the flavor to reveal another side of the amps personality. Both clean and overdrive.

Managed to play around with the LNFB today and it definitely does change the characteristics if the amp. The OD seems to smoothen out a little but the difference is very subtle. Gosh... It's a common trend with this amp... Every change made is subtle to eventually build that final tone.