Audio Note M3 Pre- Amp, Valve upgrade

I have been comparing my AN M3 Pre to some other highly regarded Pre's & it seems to be a bit lean.
It has 2 x Jan Phillips 5687wb's (I suspect they are the culprits) 1 x EH 6BM8 & 1 Unbranded 6X5.
Can any experienced Audiophile offer a valve upgrade suggestion for the M3?

Many Thanks
I have an Audio Note L3 kit pre-amp that has the same tube compliment and similar circuit as the M3. Tung-Sol 5687 tubes can be recommended along with a Mullard 6BM8. Some RCA and GE 6BM8 tubes are re-branded Mullards which can be verified if they are labeled Made in Great Britain. The Telefunken ECL82 is a good 6BM8 type, but a bit leaner than the Mullard. I also like the Bendix 5852 tube as a replacement for the 6X5GT.

Hope this helps.
My M3 is the Black Kit Pre with Noble Volume Pot & selector,
Sounds deatiled, but a little thin, doesn't throw as wide an image as some other Tube Pre's

Thanks for the advise.