Hi Michael, if you can't adjust the speed properly at the brackets, then you need to go further into the machine and adjust at the joint which shifts the idler-wheel assembly. This is the circular bit with two screws which pivots around a nut and bolt arrangement: loosen the screws, and move the idler sled backward or forward (with the two screws sliding and changing position on the two arcs) until you get a good range of + or -33 1/3 with the bracket in the middle, then tighten again.
As to the tonearm, does sound like an arm-lift problem as Harry said. Even though the Denon is burning in, and probably the phono stage too, it shouldn't sound so bad. Remember: the tonearm should be parallel to the record with the stylus in the groove. Once this is done, then use the nifty dynamic VTF arrangement on the Rega to increase mass to suit the low-compliance Denon: shift the counterweight as far back as is conmfortable on the end-stub, and then use the spring to achieve 2.5 grams tracking force. Can't do that with an RB-250! I LOVE those RB-300s, especially with MCs (here they stand up to the best, their biggest problem being they're too cheap, and too ubiquitous). However, with Grados I do believe the RB-250 sounds better, but will know more in the future.
