I picked up the TubeCube|7 a few years ago. I’d never owned a true tube power amp before. I knew that this amp would never find a place in my main system, as it only puts out a tiny 3.5 watts per channel. Yet I had a perfect use case–I was retiring my Altec Lansing computer speakers (which sounded OK, but had an annoying background hiss whenever the system was on), and wanted to up my game a bit.
Since getting the Oppo BDP-105, I had a spare DAC–a Cambridge Audio DACMagic. With a newly-built computer, I used the optical Toslink connector from the computer to the DAC, in order to isolate it completely from the computer. (No, I’m not one of those who feels Toslink is worse than Coaxial–there is a reason our Internet backbone runs on fiber vs. copper!)
I also needed speakers for the new computer desk–I located a used pair of Boston Acoustics CR65 bookshelf speakers that fit the side shelves perfectly. I knew bass would be no great shakes with these speakers, so I picked up a Dayton 6.5″ powered subwoofer which tucks neatly under the desk. The CR65s have one flaw in that there is a resonance hump at 125Hz. I had to notch that out in JRiver. Much as I hate using digital EQ, this helped clean up the speaker’s response, at least until I can get in there and brace the cabinet, and perhaps deaden the cabinet walls and/or add some fiberfill to help out with that resonance. Beyond that, the speakers have a nice midrange and the highs are easygoing.
Before permanently installing the TubeCube|7 in my desktop system, I gave it a try on the big rig. My speakers at the time were not terribly inefficient (they were around 89-90dB/w efficient), but this amp struggled to get a usable volume out of them. This amp may work well with horn-loaded speakers with high efficiency, but don’t expect it to drive most typical speakers today without running out of gas quite quickly. The bass in this case was rather ill-defined, since there simply was not enough current to drive the woofers. What also didn’t help was that there was a rolloff in the highs–everything sounded dulled through the amp. After purchasing it, I found another review online which echoed my comments, and backed it up with measurements for exactly what we were hearing.
I don’t know, then, if it was the amp’s circuitry that was rolling off the highs, or the unidentified Chinese tubes that shipped with it.
I did replace the tubes about 18 months after buying the amp–I went with new production Mullards–a pair of EL84s, and a 12AX7. I noticed the sound is slightly cleaner and less gritty. Did the highs improve? No. These tubes might have helped with added smoothness but provided no change in the rolloff.
One tweak I have yet to make is to use a crossover for the amp. The speakers are currently running full range. Blocking the frequencies below 100Hz would go a long way towards improving headroom a bit, since the amp would not be struggling with the bass. Parts Express sells a pair of crossover RCA plug adapters in various frequencies, so those may work out perfectly for this installation.
EDIT: See my updates about the amp.