Say a fella wants to build an SET monoblock based on the 300B..

Could you point him in the right direction or where to start. I see a kit on ebay but not sure if that's legit or not. I don't own the speakers yet and plan on building my own that would pair up well with the amp. Any help appreciated.  
I think you're in the wrong forum.  You might try the Amp Preamp Forum here on Audiogon, or better yet, go to Audio Asylum and look on SET Asylum or TubeDIY Asylum.  There are more knowledgeable builders there than here, by far.  More advanced is DIYAudio, a separate site.
I heard, and liked, the Elekit amp I heard at several audio shows.  This is supposedly an easy kit to build because the instructions are very clear and easy to follow. Elekit also offers several options for upgraded parts.
In truth  you are much better to let an expert build it .
Radu Tarta has been building SET amps. And DHT preamps 
for many years ,knows many secrets that only an expert  
with tubes would know. And his prices are not that much more then you can build it yourself .there are just so many things that can go wrong . The cost of the chokes ,and transformers  are by far the most expensive part of the build just check his name out in CT .he builds for the Love of the build ,profit  is not his main goal.
he has built many amps,preamps for myself and others .well worth your time to consult with him.
The kits are one way of learning but you also get what you pay for.  Why use that tube?  What are you intending to drive?  If you are looking for a beautiful sounding tube use the 45 tube IMO.  If you are going to build an amp go Class A design.

Happy Listening.
Check out the 300B set amps on He’s got several designs suitable for your use. 
The Audio Note amplifier kit is granted more expensive than the other good 300b kits mentioned above. In my opinion it’s money well spent.
1 Very sturdy and high quality aluminum chassis.
2 Very good quality standard parts. High quality power supply.
3 The option to upgrade capacitors and resistors and especially the output transformers (C-cores are available)
4Tube rectified circuit
5 Proven track record x 25 years.
6 The potential to build an upper tier high end quality 300b SET.
7 300b offers more flexibility in matching speakers due to higher power compared to the 45 and 2A3 tube options.

I agree with Charles. I have had 2 of their aamps built by their sanctioned builder, Pete Fulton, whose build quality is immaculate. I also had him build one of their tube DACs. In each case I was very satisfied and they were totally reliable. 
You Might be able to do it yourself, but for th additional 10% charge, you have the peace of mind of knowing it was done right, and the resale value will be better as well. 
I didn't know the additional charge to build the Audio Note kit amplifier is only 10%. That's seems to me a very reasonable deal unless someone wants the satisfaction/challenge of doing the assembly themselves. 
It is reasonable Charles, and if you look at their site, you'll see the beautiful job that Pete does. Besides that, the average guy doesn't have testing equipment. Also, if you want to use certain capacitors or resistors etc, they will accommodate that.
Also critical when planning for a low power amplifier is to know in advance what speakers you will want to drive with them.  It goes without saying that speaker efficiency and speaker impedance are of prime importance.
Yes, the built quality looks excellent. That is why I was surprised the upcharge for this level of professional work is so modest.

Absolutely right,  but I just assume that someone considering a low power SET would be aware of this proper speaker necessity. I sure could be wrong.
I have built the Audio Note Kits DAC 4.1x and will be building the L5 Mentor preamp soon. These are top quality kits with good instructions and support. When I finished my DAC, even though the measurements were within the ranges specified, and it played flawlessly, I still sent the DAC to Peter Fulton to verify my work. I believe Peter is retired and I think he builds these kits more for the enjoyment than the money. Also, having done a lot of the builds he is probably very efficient which saves the customer time and money. In my shop, it would would cost significantly more than AN Kits mark up. It took me 8 hours to build the DAC, at our rates that is $1000, about 30% of the cost of the kit. So yeah, Audio Note Kits is offering a really good deal for the pre-built versions of the kits. It's almost a no brainer.
Does it have to be mono blocks? If not, the Elekit is fantastic. Especially with the Lundahl transformers. 
The 4.1 DAC and Mentor preamp are gorgeous pieces that are truly drool worthy. If they were not out of my range, I would own both!
What speakers were/are you using with the  Audio Note kit 330b SET?
Based on your earlier comments regarding your happy experience you obviously have a very compatible match.
The Bottlehead Kaiju 300B amplifier was mentioned by another member. If you want to get into DIY tubes, the Bottlehead kits are a great way to go. The instructions are fabulously easy to understand and follow, and help is always available via a superb online forum. Most importantly, what you end up with is a high quality amp with excellent sonic performance. You get point-to-point wiring in the signal path, and the component quality is excellent. The Kaiju is a great 8 watt stereo amp that can be monoblocked into 16 watts per channel. I am currently running my Kaiju in stereo but hope to ultimately purchase another Kaiju kit to run as monoblocks.  I have also fallen in love with the DIY process. Building your own amps is great fun, and nothing beats listening to gear that you have built yourself. I can heartily recommend the Bottlehead kits in general, and the Kaiju 300B kit in particular.
The Bottlehead Kaiju offers the choice for either AC or DC filament heating.
Which way did you decide to go?
What speakers are you driving with you Kaiju? I have no doubt that Paul Joppa has produced a very good 300b amplifier kit.
I played with the AC filament heating for a while, then decided to add the DC heating as I was using sensitive speakers. That was a good move,  reducing the noise floor substantially. At the time I had 100 dB Zu Audio Soul Superfly speakers with a Dynaudio Sub 6 to handle the low end. Right now I am awaiting shipment of a set of Jager speakers from Bottlehead to build and try, hopefully without a sub. I am using a BeePre 2 as a preamp, and have a Rega P6 turntable with a Bottlehead Eros 2 tube phono preamp as my source for analog. Call me a DIY devotee a Bottlehead fanboy! 
Hi Scott,
I’ve followed Bottlehead’s Paul Joppa on Audio Asylum (SET Topic)  for years and have always been impressed with his electrical engineering knowledge as it relates to audio products. Very curious how the Jager will compare to your Zu Audio Soul Superfly.

Some purists believe that AC filament heating is a bit more natural but many builders disagree and say well implemented DC heating is as good sounding if not better.
O.P., since you mention you're planning on building your speakers, are you inclined to scratch build your amplifier?

If so, consider this (scroll to page 22): 

Eric Barbour's SV811-10 amplifier might look simple, but it is surprisingly quiet and has a great sound stage.  I've built 3 pair of them and have a pair still as backup amplifiers for my Elekit 300B amplifier.

I found the SV572-10 had a nicer presence, smooth and pleasant.  The SV572 triode is quite rugged.  It would take a catastrophic failure of the stage coupling capacitor or grid resistor to drive the 572 into hyper conduction.

SV572 data sheet.

Beware!  If you do build this amplifier, don't get the high gain (µ=160), SV572, you'll be sorry!  Stick with the lower gain µ=10 and enjoy.

Eric gave no parts list as he assumed the builder was capable of finding all the parts necessary.  The parts are indeed still available, just not the PCB ladened Cornell-Dubilier 121 series electrolytics. 

I've upgraded the output transformer to Lundahl (giant leap forward!), replaced the stage coupling capacitor with Jupiter oil/wax type (another notch forward) and the input stage cathode bypass capacitor with an organic polymer type.

6EM7 tubes are still widely available and fairly cheap.  You want a "black plate" RCA, there are lots of them around.

I used an ugly "potato masher" 5R4WGB (Cetron) as Eric shows in the article.  These are also readily available from many sources.

There are some updated rectifiers (SiC) that can be used in the 811/572 filament supply that give even quieter performance.  Although not necessary, I did add a DC supply to the filament of the 6EM7.

This will definitely give you an excuse to buy a set of Greenlee punches to make nice, round holes in your chassis.  (versus something that looks like a rat gnawed it out)

Happy listening