When you replace the power tubes, the bias should always be checked. Some circuits have a design (called Cathode bias, or self-bias) that adjusts for differences in tube characteristics, but I doubt very much that your amp has this feature. More likely, it has Grid bias, which does not compensate for different tubes.

Try re-installing the original 6L6 tubes, and see what it sounds like. If the amp sounds normal with the old tubes, then the new ones have different bias requirements, and the amp will have to be adjusted by a tech to work with them. Or, you can continue using the old tubes.