Sorry I should have made it clear to all, This preamp does not bypass the IEC stage. To work out a curve correction stage for each manufacture (as all heads have a different impedance) It would be a nightmare. Each manufacture uses a lot of resources to get the best curve for each of these machines. There is no guarantee there will be any benefit rebuilding it, so why change it.
If you want to buy a tape preamp with switchable input impedance you will have to look elsewhere. The original preamp has many nasty noisy op amps. They need to be eliminated.
This tape preamp will bypass the original cheap preamplifier built into your reel to reel, and eliminate those nasty op amps poor sound, and the associated noise, the only part left will be the original manufactures curve from the heads. Your reel to reel will graduate into the best playback money can buy, including vinyl. You will have that beautiful warm tube sound. I will build the reel to reel valve output, in a rack mounted case starting price of $850 finished with basic valves like (6x4 , 6cg7s , 6sn7) . If you would like me to upgrade parts, like different types of valves, duelund output capacitors etc, it's at an extra cost. For those interested just send me a PM
Ask Stevevalve what he thinks of the sound with and with out this tube stage.