As some of you have discovered, the Triple Crown runs a line level FX loop. That means +4dBu levels. It can run much higher than that around +8dBu on the clean channel with the volume maxed out. No distortion. The hi/lo channel will be less potent due to compression and clipping. There are effects pedals that can operate up to +8dBu or even higher as well as perform well in the high impedance instrument level range of -20dBu. I have long searched for an interface that would work but gave up after I tossed out all of my instrument level fx pedals. (I gave them to some friends who were just starting out). However, there is something I came across that may actually work. I was helping someone in the forums who has encountered issues with the FX loop with their TC50. It is not always mentioned in the specifications of the device it if can be used in a line level application. If it is not stated and all that is listed is signal to noise ratio or impedances of the input and output, it generally means the device is instrument level and should be used on the front end of the amp. With a short search on the subject there were two devices that may be a fix for the issue but are no longer in production. Fuchs Tube FX loop, Pigtronix Keymaster. I have no experience with either of these devices. I also did find some in other forums on this same subject suggest using a re-amp unit. So, I looked into that as a possible solution.
The key aspect of the TC amps or any other tube amp that runs a line level FX loop is that the send and return are unbalanced connections. 1/4 TS. Most re-amp units are balanced XLR or 1/4 TRS type. Some do have a compound input and output format that include the balanced XLR, 1/4 TRS but can also be used as unbalanced 1/4TS.
There is a device made by Radial Engineering. EXTC-SA that makes use of the line level send/return and runs the FX loop interface for high impedance instrument level devices. Again, I have no experience with this device. However, the intention of it is to interface a mixer, DAW or other which is line level to make use of instrument level fx units. Since the 1/4 jacks can be used with standard instrument cable 1/4 TS type, it will work in the unbalanced FX loop of the amp. Just thought I would pass that along and if anyone has used one of these units.
www.radialeng.com
The key aspect of the TC amps or any other tube amp that runs a line level FX loop is that the send and return are unbalanced connections. 1/4 TS. Most re-amp units are balanced XLR or 1/4 TRS type. Some do have a compound input and output format that include the balanced XLR, 1/4 TRS but can also be used as unbalanced 1/4TS.
There is a device made by Radial Engineering. EXTC-SA that makes use of the line level send/return and runs the FX loop interface for high impedance instrument level devices. Again, I have no experience with this device. However, the intention of it is to interface a mixer, DAW or other which is line level to make use of instrument level fx units. Since the 1/4 jacks can be used with standard instrument cable 1/4 TS type, it will work in the unbalanced FX loop of the amp. Just thought I would pass that along and if anyone has used one of these units.

EXTC-SA - Radial Engineering
The Radial EXTC-SA is a stand-alone audio interface that lets you incorporate guitar effects into the realm of pro-audio recording.