Regarding the volume drop mentioned in the previous page:
1) Neither Brian nor I have noticed a volume drop on our builds, so if it is there, it is likely something related to that particular build, wiring, or external cabling.
2) The output opamp U1b can be considered a buffer (even though it serves as a mixer too), so adding another buffer after it won't help with the underlying condition. An TL072 can drive easily a 10k impedance; your pedals and amp should be well above 100k.
3) Notice that U1a and U1b have a 47k, 15k and 4n7. These elements form a pre and de-emphasis network to help reducing noise. If for some reason these parts have wide tolerance (10% or more) volume change might be noticeable. If volume change is subtle, I'd suspect the 4n7 capacitors being different. Consider one of these components may not be correct in value, so it would help to read it with a meter. When I was starting in electronics I got a hard time from an 1N4007 diode. In the end the band that marks the cathode was printed at the anode...
4) I think the most logical test is to use this simple chain: Guitar -> Tri-vibe -> Amp with nothing else connected. Make sure this works OK before adding more pedals.