Euphoric Realism vs. Euphonic Romanticism

Exlibris, some things about this thread are striking. First, are you really running a 300B amp your MBLs? The impedance curve of the MBLs is a mismatch with zero feedback tube gear and will result in a brightness in the midrange. Here's what is going on:

http://www.atma-sphere.com/papers/paradigm_paper2.html

The MBLs are Voltage paradigm technology and the 300b amp is Power paradigm. Anytime you mix the two you can expect a tonal coloration at the very least, in this case it will be in the midrange and upper midrange- a vexing situation.

The result is likely that you will not like any preamp that is indeed neutral and musical, because it will not be laid back enough, due to the amp/speaker mismatch, if I got this right.

What you need is to run feedback so the amp can deal with the speaker. I was going to suggest that to you months ago. Either that or you need to think about a different speaker.
It would be easy to set up a feedback module that provides the gain needed to run the feedback, but IMO that defeats the purpose of zero feedback amplifiers (which is to more closely obey the rules of human hearing).

Exlibris, I bet moving the speakers into a different room had as much effect as any amplifier you tried!

Larger SETs over about 7-15 watts have trouble making bandwidth at both extremes due to the issues of the output transformer. My guess is that in this case that is helping you out. The amplifier is going to make nearly constant power on the MBL if you are using it on the 4 ohm tap. The MBL is designed with the idea that the amplifier will make only half as much power in the upper midrange where there is an impedance peak. You need feedback for that if you use a tube amplifier...

Or- as I mentioned to you some months back, you could use some form of network (to absorb the excess power), or a tone control (if you don't use feedback). From what I'm able to get from your descriptions, it sounds like you went the tone control route. As far as I can tell, it seems a happy accident, but is also why you are having such issues finding your 'holy grail'. Not much out there is going to work when you try to drive a Voltage paradigm speaker with a Power paradigm amplifier.

It is a fact though that a speaker that is designed for a zero feedback tube amp would win you more performance out of that tube amp than you are currently able to get. I think you are aware that something is amiss or you would not be on the hunt. Am I right??