I've had this line stage preamp for six months, and it's certainly broken in. I thought I'd give you my thoughts after enjoying it for a while.
TLDR: I love it.
The sound is lush, crisp and full, and the soundstage is perhaps the most impressive aspect. It's wide and deep, and when I close my eyes, the speakers completely disappear. I'm streaming Tidal through my Eversolo DMP-A10, bypassing the Eversolo's built in preamp. I like the A10, but am deeply impressed with the improvement the Rhumba Extreme provides. The Eversolo is crisp and clean, but can border on fatiguing, whereas the Rhumba mellows the sound ever so slightly without dulling the highs or mids, greatly improving the listening experience. I guess those of you with tube gear understand this, but this is my first foray into tube equipment. In addition to the Eversolo, I'm running a Parasound JC5 amp, which is very neutral, into a pair of Usher Dancer Mini-2 Diamond speakers.
The build quality of the Rhumba Extreme is impressive. It is a solid, heavy unit that looks good as well. I opted for the black faceplate, which matches the rest of my gear. The toggle switches have a nice feel. The base is a sheet of brass, and the Iso-Acoustic Gaia feet are a nice addition. The remote control is a brick, and controls volume and mute. My only complaint, or wish list item, is that it doesn't remotely turn the unit on and off. I can do that for the rest of my gear, but having ADHD means I sometimes neglect to get up and turn it off when I'm done listening.
The buying process was great. I called Backert Labs and spoke with Andy, the President and sales guy. He was willing to talk at length and answer any and all of my questions, and, it turned out, personally owned the same speakers as me. I had been willing to spring for the top of the line Rhythym, but he talked me out of it (sort of). They split the shipping cost, although I could have driven to their location and picked it up had I wanted. There is a local dealer where I could have auditioned one in person, but after talking to Andy I was ready to pull the trigger. Buying direct means I saved the 6% sales tax I would have paid had I purchased one locally. Although the units are manufactured in Pennsylvania where I live, Backert Labs is headquartered in Delaware, which has no sales tax. I had 14 days to preview it and return it if I didn't like it, but obviously that didn't happen.
I also have a year in which I can upgrade to the Rhythym for just the cost difference between the two. On the one hand, I'm having a hard time believing it can be that much better, but then I see several examples of people who have done just that. I'm seriously considering that option -- not because I'm unhappy, but if the Rhythym is even better, then, wow.
And being ever the tinkerer, I tried my hand at tube rolling, despite not really knowing what I am doing. The Backert came with a pair of Mullard 12AX7 tubes, and I tried a set of Gold Lion and GE tubes. Honestly, I'm not hearing a big difference between the Gold Lion and the Mullard. While switching tubes is relatively easy with the magnetic top tube cover, I've been listening to each at length rather than trying to do an A-B comparison. I'm thinking I like the original Mullard's best, followed by the Gold Lions, and then the GEs.
