I second Morningstaraudio; the RCA 12AX7 long black plates are excellent, and would be my first choice as alternative to Mullard long plates. I've used the Mullard long-plates (halo getter), and my recollection is that the RCA are even a touch warmer; it's a lovely warmth, though. I think if I felt the Mullard were clearly superior I'd have stuck with them, but perhaps it's time to give it another listen; I've tended to shy away from their usage due to rarity/expense. The RCA 12AX7, GE 5751 "Silver Clips", and GE 12AY7 "red label" have spent the most listening time in my tube monoblocks (phase inverter position).
I've also tried Mullard short plates, CV4004 box plates, Tele ribbed plates, Tung-Sol long gray plates, various 5751, and a couple other 12AY7. The CV4004 have their good points, but the RCA & Mullard long plate 12AX7 are flat out better. The 5751 black plates are also generally excellent, but offer a different kind of sound.
I've also tried Mullard short plates, CV4004 box plates, Tele ribbed plates, Tung-Sol long gray plates, various 5751, and a couple other 12AY7. The CV4004 have their good points, but the RCA & Mullard long plate 12AX7 are flat out better. The 5751 black plates are also generally excellent, but offer a different kind of sound.