There are better op amps.... I like the LME49990 as it is very neutral. However, one of my colleagues, who also uses Grado, prefers the OPA627AP, which is a tad bit warmer.
- RCA 6DT6 Set your LD for EF92 tubes.
- GE (Made in Gt Britain by Mullard) or Ei (Made in Yugoslavia) 6HM5 (also listed as 6HA5 and EC900). Set your LD for EF95 tubes. Avoid the small squat tubes that look like 6AK5. Look for the taller tubes.
Example listings to show what these tubes look like:
RCA 6DT6
http://www.ebay.com/itm/1-VINTAGE-RCA-6DT6A-NOS-VACUUM-TUBES-TESTED-STRONG-AUDIO-HAM-RADIO-AMPLIFIER-2-/251243281264?pt=Vintage_Electronics_R2&hash=item3a7f443b70
GE / Mullard 6HM5
http://www.ebay.com/itm/VINTAGE-GE-6HM5-6HA5-BLACKBURN-MULLARD-MADE-VACUUM-TUBE-TESTED-AUDIO-RADIO-/270927668081?pt=Vintage_Electronics_R2&hash=item3f148c2371
Ei 6HM5
http://www.ebay.com/itm/6HM5-6HA5-EC900-export-quality-audio-triode-tube-NOS-FREE-SHIPPING-/300879467071?pt=Vintage_Electronics_R2&hash=item460dd0523f
Sylvania also made 6HM5s, but I haven't tried them....
Happy tube rolling!