The 30/30 and 70/70 are the same basic amp, with the same circuit and parts quality (the "signature" versions, regardless of whether the amp is the 30/30, 70/70 or the 140/140 monoblocks, have the best parts quality), the 70/70 having twice the number of output tubes and power supplies, and thus double the power. I have heard from a good source, however, that the very best sounding amp of that vintage is the "30/30". I do not know if the alleged difference results from running half the output tubes, thereby simplifying, somewhat, the circuit, or is due to something else. The 30/30 is unusually powerful for a 30 watt amp (I know a dealer who, for laughs, hooked one up to a pair of B&W Nautilus 801's, which are very difficult speakers to drive, and managed to get some decent sound out of them and nothing blew up), but it is still a lower powered amp and will compress with a lot of speakers. The 70/70, however, sounds like a 300 watt transistor amp and will drive speakers that some mega tube amps cannot. It won't have the bass control with most speakers that a comparably priced solid-state amp will, but even with zero feedback (which is really the only way to run these amps), the bass control was always acceptable to me -- the output transformers and power supplies are high quality, the result being that the amp can drive tough loads and do a decent job controlling woofers.
If you have really efficient speakers (say, 93 db. or higher) whose impedence is not super low in the bass (not much lower than 4 Ohms at any point below 400 Hz. and no really tough phase angles), and if you do not listen to a lot of orchestral music, then the 30/30 should be fine (i.e., will make your speakers go really loud, cleanly, in most rooms). If your speakers are of more normal efficiency, however, and you listen to the most challenging sources (orchestral music on a good analog rig or good DSD), you want the 70/70. If you want to be certain to be able to drive almost any speaker to loud, clean levels, then you would need the 140/140's, which is a pair of 70/70's wired for mono operation (130 really potent watts per channel).
All of the VAC Renaissance amps (the 300-B amps) are highly transparent and require really good front-end equipment in a carefully designed environment to perform their best, i.e., if you do not intend to use them with truly top-notch partner components, cabling and home electric supply in an acoustically-treated room with a lot of attention paid to speaker set-up, your money is likely best spent elsewhere.
Hope this helps.
