The MTS concept (which is the brainchild of Bruce Egnater, who is the head of Randall R&D) is essentially a tube-freak’s answer to the digital modeling amp. The modules included for review were the Blackface, JTM, Plexi, Top Boost, Clean, and XTC (a high-gain rock lead and metal preamp based on a Bogner Ecstasy’s Red channel). A total of 13 standard modules are available, including timeless options such as Tweed, Recto, and Brown (an EVH-style Marshall tone), plus five Artist Signature preamps. Swapping a module only takes about 60 seconds, but the necessity of powering down the amp each time will seem rather Stone Age to anyone used to pressing a button to call up different amp sounds.
The other sticking point to the whole premise is the fact that many of the classic-style modules stem from nearly identical preamp designs. For example, Marshall’s plexi evolved from the preceding JTM preamp, which itself is a close copy of a tweed Fender Bassman preamp. The use of EL34, KT66 or 6L6 output tubes—as well as different speakers and cabs—helped differentiate the sonic signatures of those classic amps, and Randall has indeed gone some way toward accounting for this by tweaking the preamps to emulate characteristics originally generated in other stages of these amplifiers (such as making the Tweed’s lows a little looser than those of the JTM or the Plexi). (Note that a fuller “tube modeling” experience can be had in Randall’s larger MTS Series amps with octal-based output tubes, which are configured to accept 6L6s, EL34s, KT66s, and other tube types with a simple bias reset.)
Aside from these quibbles in the theoretical-versus-functional realm, this cute 15-watter carries many genuinely useful features. The Tube Boost function with independent Gain and Level controls offers extra overdrive saturation or a clean lead boost, Presence and Density (low body) controls expand the EQ considerably, rear-panel Power Tube Bias test points and adjustment access make this service issue very user-friendly, and a series effects loop, external speaker jack, line out, and XLR direct out (with front-panel Silent switch) provide versatility live and in the studio. The coupling of a Celestion G12M with a two-thirds-closed-back plywood cabinet promises full, but not overly tight lows—a good compromise for an amp seeking to run from vintage to modern.
I tested the RM20B with a number of single-coil and humbucker guitars, and fired up the proceedings with the Plexi module installed. Touted as a rhythm/medium-gain option, this preamp gets crunchy with its Gain set anywhere past nine o’clock (the Master was at 100 percent for full output and max headroom), and even my Telecaster caused it to break up considerably, unless I wound down the guitar’s volume. But rock heroes aren’t spending $4k on vintage Marshall plexis for clean sounds, and with the Gain anywhere between ten and one o’clock, I discovered a very satisfying small-scale approximation of a late-’60s half stack’s grind and thump with decent dynamics, ballsy lows, and enough saturation for classic-rock or blues-rock leads when you really dig in. Alongside the tube overdrive, however, there’s also a slightly grating, thudding chunk that ghosts the notes. It’s present more in the cleaner settings, but it haunts the cranked tones, too. A little of this percussive edge is a key ingredient in the classic plexi Marshall sound, but, here, it’s a shade too pronounced.
Despite my concerns about their similar roots, the JTM module sounds quite different from the Plexi. It is softer and squishier, with a slightly tizzy, gritty core when you reach the distortion point. Cranked up with Gain at about two o’clock with the Master anywhere from three o’clock to max, there’s a nicely ragged, edge-of-freakout feel that lends a very vintage vibe to the experience. Wind down from there, however, and there’s an occasional coldness and hardness that strikes me as sounding not especially tubular. It’s another preamp that breaks up quickly, too—as is the Top Boost, which gets surprisingly chewy with Gain anywhere past eight. This Vox emulation is also the sweetest of the modules, with greater depth and openness, and a more complex harmonic brew than the others. It also gets hot unexpectedly fast, with juicy lead tones that outgun the Plexi’s. For real hard grind rhythms and heavy rock lead tones, the XTC module is your dog. This one has the cascading gain and contemporary hot-mod tricks down pat, with the kind of great sustain, toothsome touch sensitivity, and willing harmonic feedback that will put a smile on the most jaded shredder’s face. XTC doesn’t even pretend to possess any clean headroom, so it’s all or nothing in this little combo.
Ultimately, the Blackface module proved best at satisfying my quest for clean. The functionally named Clean module lives up to its description, but it’s duller, flatter, and less dynamic than the Blackface, which runs ably from sparkling arpeggios to snappy twang, with bluesy lead and snarling rock and roll on tap when you push Gain past noon. Partnered with Tube Boost set for saturation, this module wrings the most versatility from the RM20B, but in discovering this, I’ve also uncovered another of this single-channel amp’s limitations: If you want to savor the vintage crunch or modern high-gain tones available, but also need instant clean options for rhythm parts, you might turn away in frustration. In and of itself, though, the RM20B is a great little gig or studio pal, with endless “tone tasting” fun to be had via its wide selection of preamp modules. g