There are a lot of blend loop pedals discussed on TB. The most popular ones I've seen are the Barge Concepts VFB-2 (and the new
VB-Jr.), the Xotic Effects X-Blender, the Boss LS-2 Line Selector, and the Radial BigShot MIX.
However, your situation's a bit odd. May I ask why you're using the effects loop of your amp? IME, it's hardly the ideal place for most of the effects you're using... which could partly explain your difficulties with tone and volume (dry outputs and signal paths aside).
Preamp signals (e.g., the "Effects Send" output of your amp) are often too hot for most pedals to handle well. While there's no risk of damage to any of your gear, you may get distorted outputs, large drops in volume, and other weird issues. Most of those pedals, IMO, really do belong at the input of the amp, and not in the effects loop.
Usually, the kind of gear that goes into an effects loop tends to be rackmount effects gear. Some pedals can handle the preamp's output, but not all pedals do this well.
The rules followed by guitarists are not quite the same with bassists...