Look for amplifiers in which the output transformer is as large as the power transformer. Another problem with the output transformers is that the laminations are often fairly thick.
Many thin laminations are preferrable to fewer thick ones.
The laminations should be much thinner than those of the power transformer. If the transformer has a 25V balanced line output winding, then it can then be used to provide power feedback to the cathodes of the output tubes, which will abate a multitude of evils. Just try to find a pair if you want stereo. Note the larger transformer to the right is the excellent large output transformer. Results of a cleanup and careful refinishing .