I acquired my 18/30 Winfield (then named the November) from Winnie some years ago, and I still really enjoy playing it. More Marshally than my Vox AC30HW, holds its own in its own way with a Maven Peal Ganesha. Mine came stock with Sovtek EL84s, which I swapped around with various NOS brands before deciding the amp sounded best with the tubes it shipped with. I like having the bright and normal volumes linked in a single channel. I also like having the 18/30 watt switching, but honestly the volume drop is not that great -- you get as much by swapping a 5U4 for a 5AR4, and more by using an attenuator.
There has been some discussion about this amp on the Univalve.net site over the years, some of which is no longer accessible due to site upgrades. Here is one thread. http://www.univalve.net/vbulletin/showthread.php?t=818 MudSkipper, a moderator on that site, had a Winfield for a long time and could probably give you a good deal of gig-specific information if you were interested.
I found Winnie good to deal with, and would happily buy one of his amps again.
I am a Winfield 18/30 owner and each time i can drop a good word here on TGP about it i do it. mMne was built for me in March 08 so nearly a year ago. It was and still is my #1 amp. To best describe, i would say it is really Marshall inspired but with el84 chime. I think it is base on a plexi circuit with some twist. The imput are internally jumpered so you adjust the bright/normal channel volume to taste. It is a Non master volume amp, so if you are not familiar with that (like i was) it takes time to get use to it but it is great. The amp has great dynamics and respond really well to the guitar volume, because of the simple signal path. I play min now with an attenuator because i wanted to get the tones i liked at home too. The switch for 18/30 like the OP mentioned doesn't cut volume that much but change the caracter of the amp, i really start to appreciate it just now. So i think that Plexi meets Ac30 is a good description but it is more then that. If you have any question you can PM me and i'll answer the best i can.
Plus i need to add that Winnie has been nothing but great in my experience with his product so i would buy another amp from him in a flash (actually thinking of a Cyclone...)
I had a head a couple years ago. Nice amp. I got it for home use but because it's a NMV, I had to crank it too much and I didn't like any of the OD pedals I had. So I ended up selling it. Plus I'm not that crazy about El84 tone.
That amp started as an Aiken "November" design and evolved from there, so it has some great heritage. It's a good sounding Marshall/Vox blend and can be easily tweaked for more or less gain with tubes or a couple of component mods. Very giggable, can get quite loud if you need it.
I have a mid 2008 Winfield 18/30 as well. I would agree with fabulusfab777 that it is "Marshall inspired but with el84 chime". It can get very loud in both 18 and 30 watt settings. If you want to get it sounding dirty at less than very loud volumes then an attenuator seems to work well. I use a Weber MASS lite so that I can get the amp singing at more reasonable levels. It works well as long as you don't try to attenuate it too much. I would probably just get the 18 watt only version if I were to get another one. It is cheaper, lighter, and still loud - just with a bit less headroom.
There are a couple other things I did to bring on some power tube grind a bit earlier. I changed the 5AR4 rectifier for a 5V4 (tried a 5U4 as well) to drop the voltage a little. I also changed V1 to a NOS 5751. Note that V1 was very sensitive to tube choice, and I had to try several tubes (that worked fine in other amps) before finding one that didn't significantly increase the noise level. I have also switched out the PI for a ANOS 12AT7. I liked the amp with the original tubes, but like it even better now, and can get some earlier beakup. It sounds great through a greenback into an oversized 1x12 closed-back cab.
I will say that it is not the quietest (noise wise) amp when cranked up. I have others that have a lower noise floor, but it is really only noticable when playing on my own when either volume is above noon. I talked with the builder about the noise and he felt it was normal for this type of circuit. He was also helpful when asked about using different rectifier tubes, and was very friendly.
These are nicely built amps, with good components at a reasonable price. I appreciate that the head is relatively small, compact and light weight. It sits nicely on a 1x12 cab. The transformers are not MM, but they are beefy and the amp sounds great so I don't particularly care.
I would certainly consider buying another of Winfield's products.