Since you have 8 tubes, the odds of another one failing is greater than zero. Buy another set of matched quads of the same brand and type as the originals and keep the 3 old tubes as backups. I wouldn't put Mullards in one monoblock and JJs in another, that would be too weird sounding.
More info about the amps and current tubes would be helpful, someone here on AC may have the same amp and know if you can stick in any old EL84. The boxes the tubes came in might have the test numbers, I don't know how close the numbers have to be to qualify as matched but you may find a single tube with test numbers close enough.
If the amps are uber expensive then the manufacturer will have a record of the 2 sets of matched quads and help you buy a single tube. I borrowed a pair of monoblock amps not too long ago and something was slightly off about the sound, my Van Alstine Set 400 amp imaged better. The effect was subtle but led to my theory that monoblock amps have to be precisely matched to each other, any deviation in frequency response will smear the image leading to a more diffuse sound stage.
