Here's an empty moving coil phono step up transformer box for use with your RCA MI-12399 transformers. This box will also work with Harman Kardon XT-2, XT-3 and Bogen T-155 transformers. I do have a listing for an identical box engraved for the HK transformers. If you'd like yours engraved for the Bogen, please just let me know. It may add 2-3 days to the handling time. By default I'll send one labeled MI-12399.

This box has a gain switch on each channel, allowing you to use the transformer's full primary coil, or the center tap to double the ratio and voltage gain. 

There's also a ground switch that allows a few different options for grounding the aluminum case. You can use "L" to ground the box to the left RCA audio shield, but keep the two audio shields separate from each other, or you can use "L+R" which connects the box to both shields and thus also joins the shields together. You can also put it in a center position which doesn't join the box to either audio shield. This position allows you to keep the audio and chassis grounds separate, as they come out of most turntables, all the way up to the preamp. Because in the center position the chassis isn't grounded to either RCA cable, you do need to run a separate ground from the SUT to your preamp ground screw in this position. Whichever position you put the switch in, you should normally be able to attach your TT's tonearm ground wire to either the SUT ground screw, or the preamp screw, whichever you prefer. In general you don't really have to worry too much about what the ground switch is doing, just hook up the SUT however is convenient and then put the switch in whichever position gives you the lowest hum.   

This box is priced a little higher than the other empty transformers boxes I sell because of the difficulty and expense of finding good quality 9 pin sockets. I only use NOS Amphenol sockets for these, never used ones. This is a newly made box. It has been tested and is guaranteed to work, assuming your transformers are also working fine. Let me know if you have any questions. Thanks!