fire spark in 5AR4 rectifier tube for PrimaLuna Pr

I tried switching the tubes, and the spark appears on one of the tube, so, it is in fact the tube has gone bad.

So, now, can I just replace one or do I need to replace both with a matching pair?

just replace the bad one...they don't need to be a matched pair. Although I'd probably replace the pair w/a nice pair of matching NOS & then keep the good single for a backup. enjoy!

Just a little FYI; The best sounding, most reliable and longest lasting rectifiers out there, are the Blackburn Plant, Mullard tubes(the earlier the production-the better). Unfortunately, do to all of the aforementioned facts; they are also the most expensive. ie:(http://www.ebay.com/itm/Mullard-Blackburn-Branded-Amperex-Bugle-Boy-GZ34-5AR4-Rectifier-Matched-Pair-/220910480753?pt=Vintage_Electronics_R2&hash=item336f4a6d71) (http://www.ebay.com/itm/vintage-Mullard-Blackburn-5AR4-GZ34-NOS-Hi-Performance-rectifier-tube-/200687014748?pt=Vintage_Electronics_R2&hash=item2eb9e0d35c) (http://www.ebay.com/itm/NOS-box-Mullard-GZ34-5AR4-double-halo-tested-1958-Blackburn-never-used-/120827434079?pt=Vintage_Electronics_R2&hash=item1c21e0545f) Personally(being your preamp is a dual-mono design), I'd replace both rectifiers, with a matched pair, to insure a balanced presentation from the two channels.

Yes, mullard blackburn tubes are too expensive. Any cheaper but good NOS alternatives? Also, if I don't want to spend too much, is it still worthwhile to get 1 Mullard Blackburn? Or better to get a match pair of new stock?

GE, you can drive nails w/them, try Jim McShane Tubes or Andy at Vintage Tube Services.