Denon DL-103 -- Was it a mistake?

Muddy bass could be a sign of excessive feedback from the speaker. You might want to move the TT around a bit to see if the bass improves. Another way to test it is to lower the cart on a record without turning the motor on, than slowly turn up the volume and see if you get any feedback.

Well I guess I'll find out when the 640P arrives. I never heard anyone say that it was bright before, after reading several reviews. I wish I would have known that beforehand, because my system is definitely not dark. I'd say that it's fairly neutral as is. It may be possible to mod the 640P to change the loading on the MC section. I'm not afraid to use a soldering iron, I just need to know which caps/resistors to change.

I could also try moving the TT around, because it is fairly close to the speaker. I never noticed a feedback problem before, but that was with a higher output MM cart. Maybe it is more sensitive now with a low output MC.

My experience....

I currently own six DL 103 series carts ( 2 stock DL-103, 1 stock DL-103R, 1x Dl-103R SS retipped, 2x DL103 with wood body).

All I can say is that the arm needs at least 14-16 gms minimum effective mass in order to hear what the 103 series are really made of excluding the headshell weight. On the stock 103's currently installed in my system, I found the Orsonic AV101-S (16 gms) and Yamamoto (10.4 gms) to sound the best loaded at around 40 to 100 ohms in 26 dB SUT gain (1:20) with VTA a tad high (tail up a bit). As you can see, using the two headshells results in an effective mass in the range of 24-30 gms effective mass.

I tried unipivot arms in the 14 gms and less tonearm mass before and I was not pleased at the results even if I add weights on the headshell.

On the two wood body carts I have, I found that it sounds good even on the SME 3009 S2 Improved arm (9.5 Eff. Mass not including the original SME headshell which is about 6~7 gms)but really sound much better using a heavier headshell (Orsonic or Yamamoto).

So, in summary, try using an arm with a total effective mass of 23 gms. and above and I assure you from my experience, that you will hear a new sound on your DL103 compared to what you have now.