Thank you for the extra information. I think you may have overdone the googling though, and confused what you have read about diagnosis of low PSA producing cancers and those which have become androgen independent after a period of hormone treatment.
There are I think 27 different types of prostate cancer, only a handful of these do not produce much PSA even at an advanced stage. Allister mentioned small cell carcinoma which is unusual because it doesn't respond to hormone treatment and there are types where the PSA goes up as normal until the very late stages when it inexplicably drops very low.
There doesn't seem to be any reason to think any of these circumstances apply to you at present. The cause of your backache is more than likely to be as a result of having an enlarged prostate. I assume you have been prescribed some tablets to reduce your prostate size? If you have had biopsies and scans which have come back negative for cancer, and that has not reassured you, then you should really ask your urologist whether you could have a more detailed biopsy under general anaesthetic.