Early last year, I picked up two new pieces of equipment: a VTA-70 and an SP-10 preamp.
I've owned tube amps before: a Conrad-Johnson MV-55, a Cary SLI-80, and a stock Dynaco ST-70. At one point, I decided that I would never mess with tubes again (don't ask, I don't remember why), but that proved to be the wrong decision. So at that point I picked up the ST-70, put new input jacks and speaker outputs on, and used it for the next several years with a passive front end.
After a while, the ST-70 started to show sign of wear; either some of the soldering I did was not that great or...well, who knows? At this point, it had been many years since I picked up the soldering iron, so I decided to get a pre-made amp, and decided on a VTA-70. And it sounded great; but I started to wonder what it would sound like with an active preamp. So I picked up an SP-10 preamp.
I should note here that my VTA-70 has the cap upgrades, and the SP-10 has Auricaps in it. So neither of them are stock. And, as long as I'm disclosing stuff...I did a little tube rolling on the VTA-70. I know that Bob Latino suggests that no tube-rolling be done before running the VTA-70 in for about 200 hours, but also, I have to admit that after about 100 hours, I realized that the Valve Art EL34s that shipped with the amp were not to my taste. So I put in some SED =C= 6L6GCs. I also pulled the "Realistic Lifetime" rectifier (probably a Matsushita) from my stock Dynaco and put it in the VTA-70.
Here's my first impression, from when I was using the passive front end: the VTA-70 is not only different than the stock ST-70, but different from any other tube amp I've ever had. Both of the other tube amps I've owned imposed much of their own character on every recording. The VTA-70 gives a beautiful tubey character to everything, but there is no mistaking one recording by a given artist for another by that same artist. Yet everything is very listenable; once the power tubes settled down a little, there was no harshness in any recording I listened to; even some recordings that were mixed very harshly settled down a little. Note: by 'tubey character', I don't mean lush and diffuse; I mean that there was none of the artificial treble and bass emphasis that I've come to associate with much solid-state amplification (even some very expensive solid-state).
Since I was so happy with the VTA-70, I thought it would be nice to try a VTA preamp. I chose the SP-10 because it had the features that I needed: a balance control and a tape out for a headphone amp. The SP-10 is the first active preamp I've had in about 10 years. I don't have a lot of audiophile vocabulary for the difference it made; mostly, there is more weight and presence to the music. And though the passive front end was a little more detailed in an odd way, the music now has a coherence that it didn't have before. Rock recordings _really_ rock now, and I keep having those "wow, that was great!" kind of moments. There are also things I'm hearing that I don't think heard really well before, like Robert Cray's beautiful guitar tone, or how great the Beatles' harmony vocals really were.
For me, the VTA-70/SP-10 combination passes the most important tests of a stereo system: I don't get fatigued after listening for three or four hours, I don't have to give up features I want for a great sound, and it does all types of music well.
If I'm unhappy about anything, it's that I didn't get a kit and build the VTA-70 myself. But that's ok; I have a stock ST-70 that I'll be upgrading with the tubes4hifi octal driver board once I feel I've done enough soldering practice. I'm also a little worried that after a while, I'll get the uncontrollable urge to check the SP-14.
I hope this post is useful to someone. I would not hesitate to recommend either of these components to anyone who likes to listen to music.