Passive tube stage/non-DAC device?

The Musical Fidelity X-10D is an even order harmonic distortion generator, hence the "warm" and "rich" tubey sound. If i can recall correctly, distortion was somewhere around 10% when run through one of these devices.

Rather than investing in a device of this nature, try working out the problems in your system. Otherwise, your trying to stop the bleeding of a gaping wound by filling it with sand. Sure, the bleeding will stop but you've got a festering sore that simply won't get any better on its' own. Sean
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There is also the Z-man Analog Signal Enhancer (ASE) which uses a single 12AX7, has a decent power supply (not a wall-wart like the MF X10-D) and an on/off switch. I've used mine in some high-res systems with very good results. Currently I use one in a system with Magnepan MG-12s and an InnerSound ESL amp. They can sometimes be found used for $90 to $120, but I don't think they are currently in production.

I just reread your post and realized your were looking for tube devices. The Taddeo units are not tube devices, but effective.

Check out the Musical Fidelity X-10D and the Taddeo Digital Antidote I and II. Happy listening.