So....the new loop uses an NOS 12AU7 high current tube. This tube is way more capable of driving long cables with much less High frequency roll off due to cable capacitance. The signal to noise ratio is also much improved from the 5751 based design.
The send control on the back of the amp has always been able to be used as a master volume control for the entire amp, with 'unity' being at the 11 O clock position. The new system also allows this control to be used as a master volume for the entire amp, the 'unity' position is now 2 O clock, giving much more available range to bring the overall volume down if required.
This new loop will feature on all amps from serial number 151106 onward.
Please note it is not possible to just put a 12AU7 in the current loop, it will potentially cause damage.
I will be taking amps in for upgrades to this new loop from the beginning od December. Cost is $150.00 plus shipping and includes the NOS 12AU7 as well as an entirely new loop circuit. Please email me at firstname.lastname@example.org to schedule your amp in if you want to take advantage of this new loop.