I know what it's like when you can't find what you want. And the reality is that there are not many comparative studies of physiotherapy-specific approaches/treatment/techniques. Perhaps this is something that you could one day contribute to?
I have found one study that compares PNF and PNF+TENS for hamstrings flexibility in healthy males. If you have access to PubMed, here's the link (your university librarian may be able to assist you in finding):
There is also another link here to a piece on knee rehab which refers to TENS:
On another point, placebo is not a "bad" thing. Some people call it "expectation" and it's part of the healthcare relationship that we have with our patients/clients. If we develop a good rapport, and provide our patients with an expectation of improvement from physiotherapy, then everything we do can have a potentially positive contribution to outcomes. Surely that's something we can all strive for?
My last comment is reserved for those who hold the opinion that a lack of evidence means that something doesn't work. It simply means that no-one has yet done the research and published it. So, if you cannot find evidence, it does not mean there is a lack of efficacy; it just means there is a lack of evidence (i.e., any evidence, positive or negative)!
Keep studying hard and let us know what you find out.