I'm the proud 'new' owner of SE84CS #152 & #260.  They are operating as mono-blocks, and sound very good, but I'm not sure they are very well matched, and there is a phase 'oddness' to the sound, though it's more noticeable when warming up.
To match the levels, the older model has it's pot turned down to about 93% (measured resistance), but the older one still sounds more 'juicy' & the newer a little more flat.

I've checked voltages & components all look (& measure) to be the same & in good condition & swapped the valves several times with the same results.  The only difference I've found is the output transformers.  The older ones primaries measure 12H & the newer ones 22H.  The corresponding numbers on them are EM0493 & EM0406.

Is it possible to buy a suitable pair of transformers to match the two amps?  I have to say I'd prefer them to sound more like the older one, though if this isn't possible, having them both the same as the newer one would be ok.
Incidentally, I don't live in the U.S. so shipping back to base would be difficult (& costly!).

The EM406 is the original transformer for the SE84C and SE84CS.  It is set up to drive 4 ohm speakers.  This transformer has a 9800 ohm primary.

The EM493 is also known as the "EX transformer" is custom installed when a customer is driving 8 or 16 ohm speakers.  This transformer has a 3300 ohm primary.

Steve