Tubeyist sounding Pre-amp?

Obtain the tubed pre of your choice, then load it with either Mullards or Brimars(British valves/NOS, NOT repops). British tubes are generally quite euphonic(warm/caramel flavoring). There are a plethora of excellent 6922/6DJ8/E88CC types out there that should even enable you to adjust your Sonic Frontiers pre to your tastes.

@Randy- Did you read my previous post? It will cost much less than $500, to experiment with your Sonic Frontiers pre and British tubes. Just add two matched, LOW NOISE tubes in each gain stage, until you obtain the presentation you prefer. Amperex tubes will also add some warmth, though not as much as the Brits. These recommendations come from over three decades(going on four) of tube rolling, in a very wide variety of components/systems.

Have you had Sonic Frontiers check your pre?   It's a pretty good piece, and you'd be hard pressed to find an equivalent that will outperform it, with the right tubes.  At least, not for $500.00.

@Randy-  Here's a bit of info that may be of interest to you.   Read the section, toward the end, that's headed, " MULLARD, GENALEX, BRIMAR, and other British made NOS".  (http://www.audiotubes.com/6dj8.htm)

@bpoletti-  I'm of the exact same perspective, with regard to listening tastes.  However: as one that's been in the business of helping others either produce their own, or- enjoy the music of others, for a few decades now; I've never felt it my place to adjust another's listening preferences.  Even if it's diametrically opposed to my own.   I like to leave them smiling!

@toastado- If you read the info in my posts, the British flavor and sonic qualities of the tubes mentioned extend across their lines of nine-pin miniatures(12AU7, 12AT7, etc). Try a pair of Mullard( 1960s date coded, Blackburn) 12AU7/ECC82 valves(if that’s a Sphinx). OR- If you’re feeling especially ritzy: 7316 Selected Grade early Amperex/Philips made, RARE D-getters(1950’s date coded, Holland). Their halo-gettered, 1960’s 7316 isn’t any slouch either. They may be the last tubes you have to buy for your Rogue. ie: (https://www.audiogon.com/listings/tubes-amperex-holland-7316-ecc82-12au7-medical-grade-long-plate-d-...) for the pics only.

Do yourself a favor and sent the Sonic Frontiers back to the manufacturer, at least for an estimate.   They will use replacement components that will not degrade the unit's performance.   Just getting the piece operational should not be the goal.