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.
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.