Reproduction of acoustic instruments with tube amp

Yeah, it's worth giving it another try. Your ancillaries are good stuff. The reasons tube amps can sound best with vocals are generally the same reasons for the way they handle acoustic instruments. I just call the whole gestalt an ability to sound "natural", having a lot to do with the way the harmonic structure is preserved, and the way images are cast in their acoustic environments. SS bass slam and weight tend not to matter as much as these qualities when it comes to vocals and acoustic instruments. But in addition to trying better tubes themselves, it seems to me that you don't favor a "tubey" characterisic warmth that might compromise separation and definition. This would point to trying an amp (still push-pull, probably not in triode mode if switchable) with a large, wide-bandwidth output transformer (for un-rolled bass and treble extension), possibly running higher-output tubes than your VTL's EL-34's such as 6550's (and maybe more than one pair per channel?) for higher undistorted output capability (in conjunction with a beefy power supply), and possibly using an ultralinear output topology and somewhat greater negative feedback (both as opposed to the VTL) for lowered output impedance (less frequency response deviation into a 4 ohm load) and a higher damping factor (increased bass control). If you spend the money to get these things, you should also get passive parts quality on par with your Plinius, bettering transparancy and detail. The result may be a sound that, within the scale of tube amps, is more "dry" and less "luscious", with images that more favor focus over breadth and a soundstage favoring intimacy and clarity over distance and reverberance, and a small-scale dynamic character that may lean less toward "freewheeling" and more toward "controlled" traded off against increased large-scale dynamic resistance to sounding "amorphous" during complex passages. Obviously, many folks prefer tube amps for just the qualities opposite these, but you may not. Then let us know how things turn out. Good luck!

VTLs are not bad amps at all but they have a tendency to "eat" tubes. They do not benefit too much(as to the value invested) from placing some fancy tubes on them so stay with stocked ones and save bucks. I strongly recommend to have a tube tester for VTL amps and test tubes every month+- depending on how heavy you listen. The bias circuitry can suffer from the bad tube and be partially destroyed. Biasing is necessary to perform at least twice per month. That is the other way of keeping track on performance stability. If suddenly you notice that one of the tubes is out of bias by large values you'll need definitely to check it. If you don't have a tube tester you must have an extra tube to place instead just in case and than somehow test the extracted one(it may be OK after all).

Zaikesman, thanks for the great advice. This is exactly what I wanted to know.

It's always been my understanding that an amplifier is supposed to reproduce its input signal regardless of whether it was originally an acoustic instrument or not. Acoustic instruments are not inherently more complex than their electronic counterparts. A Les Paul thru an original Mesa Boogie should sound different than a Telecaster thru a Vox AC30. Electric pianos do not sound like real acoustic pianos, but a good amplifier (tube or solid state) should be able to reproduce the sonic differences between a Wurlitzer and a Fender Rhodes.

Dburdick, in direct response to your question, give the Mesa Baron a try. It's within your budget (used), produces relatively large amounts of power and gives the user the ability to tailor the sound. Best of luck.

61, of course you are right about an amplifier's being supposed to reproduce the input signal it's fed - in theory. But in the real world, no amplifier can do this perfectly. So amplifiers introduce colorations, have strengths and weaknesses, sound different from one another, and may have characteristically indentifiable traits within design types - meaning that certain design schools may tend to broadly share some relative strengths and weaknesses.
By the same token, different types of music will tend to differ in the characteristic demands they place on the equipment reproducing them. Tube gear is commonly considered to be capable, when well-executed, of standard-setting reproduction of pure harmonic overtones and sonic recreation of space and objects. These strengths correlate well with the demands generated by naturally recorded and live-captured acoustic music and voice. With electronically produced and studio recorded and processed multi-track music (like most rock), the demand for the above qualities can be lesser because little genuine space or acoustically natural harmonic overtone content may be present. The traditional solid state virtues (again, when well-executed) of separation, definition, and bass weight and slam can be of more relative importance here. Obviously, and as previously intimated, these are broad generalizations, and quality amps of all types can do a capable job with many sorts of music types. But differences will still exist, and an audiophile's preference in types of music listened to will often have an impact on which set of amp characteristics are weighted with the most importance for the job.