You can use a Line Level Source Selector1. Connect the HT receiver LFE to Line Level Source Selector one of the input red rca jack, white rca jack leave open (no connection),
https://www.amazon.com/Technolink-TC-721-Turntable-Source-Selector/dp/B078RV1KBX
OR
https://www.amazon.com/Automatic-Two-Way-Source-Selector-Activated/dp/B00L9YVL90
2. Connect the tube amp pre-out to Line Level Source Selector second set of input rca jack red to red, white to white,
3. Connect the Line Level Source Selector output to SB-2000 rca inputs red to red, white to white.
If using Automatic-Two-Way-Source-Selector
1. Connect the HT receiver LFE to 'A' input red rca jack, white rca jack leave open (no connection),
2. Connect the tube amp pre-out 'B' input rca jack red to red, white to white,
3. Connect the Automatic-Two-Way-Source-Selector output to SB-2000 rca inputs red to red, white to white.
When tube amp is in play, SB-2000 will automatically connect to it.