Nice progress on your 300B interstage amplifier. I am always pleased to see another diy'er who takes as much of an interest in proper wiring practices, grounding practices and overall neatness inside the hood. It's my firm belief that a qualified tech or the manufacturer should be impressed if they look under the hood and one should be able to 'understand' the circuit if you follow the path/traces.
I like all the upgrades you have implemented, particularly the Maida regulator (which is a staple on any of my tube builds), the ISS board (a no brainer), as well as the Tent Labs regulated filament supplies for the 300B's and Pete Millet's regulated supply for the driver stage. These are areas where most manufacturers will skimp and in my opinion, the sonics of the 300B drop down to mid-fi level.
One area I would consider experimenting is in your balanced input connection. In my experience the Jensen input transformers and Lundahl Cobalt Amorphous core are quite excellent but still lagging behind Tom Christiansen's Universal Buffer design which is basically a straight wire with gain. When you think about it, you really want the cleanest design for the small level signals prior to applying gain. Or else , that level of detail is lost forever.
The other area I would consider experimenting is the speaker protection circuit. Most I have played with including the ones from the DIYAudio store affect the sound. Moreover, they are prone to failure and arcing compared to MOSFET based designs. I would definitely consider Tom's Guardian-86 if compatible. That being said, this is the first time I have seen a speaker protection circuit in a transformer coupled tube design which is interesting. I would've figured the output transformer would provide some degree of protection should a disastrous DC mishap happen, although having a delayed startup is always a great thing which is one of the reasons I use it in cap coupled solid state designs and/or a tube design. The other thing you can try is a high voltage B+ delay and if you vary the time constant to 30 seconds to 1 minute, that should take care of any initial noise issues as the amp powers up. It’s just that many speaker protection circuits aside from Tom’s can be an impediment to the sonics since they are directly in the signal path.
The use of higher end passive parts, i.e. caps, RCA's, XLR jacks, IEC's etc....is nice icing on the cake. But like most tube designs, much of the distortion is going to be from the output transformers (especially in the bass and at high output levels) but it looks like you have invested well here. I am looking into Monolithic Magnetics myself.
Carry on! You have a $20K tube amp in the making! Very impressive!