Hi Rob,
Nice one. How much did you pay for the quad? IME, NOS/NIB ones are normally around £70 a piece (or the equivalent in dollars).
I rate the 5691s highly, especially in terms of construction and reliability. I'm also a fan of Melz 6H9Cs. I've used both in my Croft. However, there are other 6SL7s that could also be considered as 'the holy grail'. If you get an opportunity, try some 1940s vintage [*must* be that, not later versions] black glass Tung Sol 6SU7GTY or Mullard ECC35 ['tall glass' with short brown base, such as shown here: https://www.ebay.co.uk/p/1008033773?iid=383269849766 ) both of which IMO would give your 5691s a run for their money!
However, as I've said to Andy elsewhere, much of this comes down to the circuit that the valves are being used in and your personal sonic preferences. Enjoy!
Marco.