General Opinions-- Zu and Manley?

I prefer tube rectified in general.

If you want an extremely outspoken declaration of love for the Manley Neo-Classic, see Arthur Salvatore's website, http://www.high-endaudio.com/ He is the crazy Canadian guy whose views seem to be based upon having heard a grand total of four systems, and who gets into cat fights with the editors of Stereophile and the Absolute Sound (why they engage him is beyond me).

You have to be careful to source Electro-Harmonix tubes from a reputable seller with proper testing equipment, as quality control could be better - the ones that are well built work fine and sound okay, while those that aren't can die quickly and tend to hum.

You should indicate whether you primarily seek a push/pull amp or a single-ended amp, as they are of course two very different beasts. There are hundreds of SET amps that use 300B's for output tubes that vary in price from $500 to $50,000 - I don't know where to begin to start with a recommendation except to say that really heavy, expensive ones are generally really heavy and expensive because they have big power supplies and top-notch output transformers (which are heavy and costly). As for lower powered push/pull, the VAC Renaissance 30/30 is superb from the perspective of build and sound quality, and for higher powered push/pull, the same can be said of the VAC Renaissance 70/70. The "Mk. III" and Signature versions of those amps autobias the tubes and have a "sentry circuit" that shuts down any output tube pair that goes out of spec - these features take the hassle and risk out of owning a tube amp (the risk and hassle of finding good tubes to run with a given amp remains).

I own the Manley Neoclassics and have heard a few other 300b amps as well. Sonically I find them to be more neutral sounding and balanced from top to bottom than typical 300b amps (at this price point). This is not to say that they don't have the beautiful midrange that 300b amps are known for (they do) but rather that they also have surprisingly good bass performance and overall drive as well. In SET mode they definitely sound more powerful than their rated 12 watt output. I listen in SET mainly which is plenty for my 93db efficient speakers but PP mode is also very good. They are quiet amps too - you have to put your ear right up to the tweeter to hear a very faint tube noise.

I found that the Neoclassics improved significantly with NOS and upgraded tubes. The biggest improvement was replacing the Electro 300b with Sophia Carbon plates which added further dynamic headroom, bass control and overall refinement while keeping a neutral character to the sound. I replaced the rectifier tubes with 50s RCA 5U4GBs and then went to 50s Tung Sol 5U4GB which I like better. On the input side I replaced the OEM tubes with NOS JAN RCA 6SL7s and on the driver side I use either Ken Rad VT 231 or RCA smoke glass VT 231 (I prefer the more organic sound of the RCAs). You might also consider a set of Slyvania VT 231 (tall bottle). With these amps you can kind of "tune" the sound to your exact liking by swapping out driver tubes.