In a passive pre you have a set of input sockets and an input selector switch.
The output from the input selector switch goes to a Potentiometer or stepped attenuator the out from that to a set of output sockets
Take another set of wires from the selector switch output and take them directly to a new set of output sockets, this second set goes directly to your R2R input, the output from your R2R can go back into one of the source selector sockets.
This will give you full line level input to your R2R so if you are recording from a DAC or a CD player you will be getting approx +4db / 320nWb/m level if your recorders can handle this input level you will get a very clean recording
older tape loops had buffers and attenuation but just sound dull IMHO
Alternatively just do what I do and plug the output of the DAC/CD/Phono directly into the R2R input for the best recordings
Alan
Turntable - Garrard 401/Jelco 750L/Ortofon Kontrapunkt B, Pioneer PLC 590, Micro Sieki MA505 , Denon DL103R - DIY Paradise Phono stage - Reel 2 Reel Studer A810, Otari MX55,Tascam BR20, Revox A77, B77, PR99, TEAC X1000 & 3440, Digital HTPC / Young Dac - Preamp - DIY B4, 821, Power Amp's DIY Avondale NCC300 Mono Block, Speakers Wilmslow Kit Volt BM220.8 / Scanspeak D2905/9500