It is possible that you have a lemon. Mainland Chinese electronics production is not as mature as, say, Japan's. I have had very good results with mine (although admittedly I have only had it about 2 weeks). Very smooth, non-fatiguing sound. Do you have a Bertrand unit or a grey-market? The Bertrand's seem to me to be pre-modded, what with their claims of NOS tubes, etc.
Cheers