I think the mullards have very smooth mids and really clean, crisp highs. Telefunkens seem to get a little better bass. In my experiences, 5751's rock a little better. They do not smooth over any sound. 5751's can handle more plate voltage which is why the can "crank" a little more. CJ pre-amps always came with 5751's and I hated it. They would cook mullards but I felt they sounded a little more "tubey" with mullards in them (I've had a PV-5, PV-9 & PV-10).