regarding 300b - I have 4 300b in parallel for 60 watts continuous using Emission Labs 300b XLS that have been solid for 7 years now with significant use. My preamp takes 4 300b as well but are not monoblocks. I run Kron's for those. Personally I like the Kron's a ton. Price for benefit seems best value. My experience with Kron's is they have an unusual high failure rate in the first 200 hours then are solid for the foreseeable future. Kron stood by their tubes and sent me replacements.
I do know of 6 Takatsuki TA300b that are up for sale with under 50 hours on 4 of them. They were purchased by my recommendation and the owner, David Beetles of Hammertone Audio, preferred the Kron's. Contact him, if interested. He will give you his impressions directly and a good deal if interested.
WE has released their 300b. I am on the sidelines for now.