Was the test for grounding pin 2 done with a short link from the pin 2 terminal to the ground end of the 1k8 cathode resistor of V2a ?  Is the hum drop on CH.1 and on CH2/CH3 ?

Is the hum killed on CH1 if you lift one leg of the .07u cap coming from the relay to V1b?

Can you connect another 15 to 100uF cap across the 1k8 cathode resistor on V2a?

Using the 'Preamp Part 1" schematic for reference in the link you provided, can you temporarily short the grid to the local cathode of V2A, to confirm it is not a problem from then on?

Perhaps then temporarily short a further back point like the wipers of CHx Gain when they are selected.

Similarly you may be able to short across the tone pots (top to bottom of series string of pots on CH1), and across the 2M2 loading resistor for V1A.

The initial aim being to include/exclude certain sections of circuitry.

Perhaps the output transformer primaries are both gone, assuming there is B+ at W102 now (given the new power transformer and B+ diodes) and it's not a Standby switch problem instead. 

Maybe best not to have power tubes in with screen voltage but no anode voltage.

Interesting that the PT was taken out.  Did you confirm the fuse values were as per the schematic (500mA for F102 on HV secondary, and 1.6A for F201 on primary) ?

Was the mains or HT secondary fuse blown?
Are the power transformer windings ok?

I haven't come across another vintage USB interface device that groups together the same suite of benefits, and be sold 2nd hand at a low price as the EMU range of USB devices.  Hurdles can arise with vintage gear like this - such as with software and running on latest PC operating systems - luckily the beta driver for that device runs on Win10, and REW has no problems using ASIO driver with it - but caveat emptor with trying to use any USB or other PC interface device.  However this all relates to a PC connected tool, not an Android as is the topic of the thread, so a bit off piste.

I looked through the Android Micro-USB OTG topic the other day, and hope to confirm that 5V is available from that port without hassle of my test Nexus, although it seems that not all Android devices may allow that feature, or allow it in the same way.

I lucked upon an EMU0404 USB interface for just a few $.  It's key advantages are a 1Megohm unbalanced input with wide attenuator/gain stage, and a wide range headphone/line output, and a very low noide floor, and a 192kHz sampling rate.  Add a standard scope 10:1 or 100:1 compensated probe, and a battery power supply, and a battery powered computer or USB isolator, and REW software, and you get a top class spectrum analyser for audio for under $50.  Other software can be used for time-domain scope waveforms.

Using an interface without all those attributes forces compromises, and may be not so easy to work through to a practical outcome.  I've tweaked $1 USB soundcard interfaces - not too difficult, but tweaking requires the MIC low impedance ADC input to be raised to 1 Megohm if a standard scope probes is used.  A 100:1 probe goes a long way for protection.

An Android device would likely have a similar MIC input issue - I just updated an old Nexus with the aim of setting up a BNC probe input adaptor if I get the chance.  Could certainly make a convenient portable tool.  USB OTG allows a 5V supply for powering any interface opamp.

I'd be a little cautious about using a an expensive android phone for general use, as it may be easy to fry the microphone input. 

But if you have an out of contract old phone gathering dust, or other hand-me-down device, then go for it.  Often a 10:1 scope probe can be connected to a MIC input, and bandwidth may be tweakable by adjusting the probe compensation capacitor (even to the point of removing any fixed cap in the probe).

Ciao, Tim