What is the appeal of the Denon 103 cartridges?

IMHO it takes a heavy arm for that low compliance (I use a Jelco 750 for them).  Proper use of a step-up transformer really helps as well, and there is a lot of disinformation around.  Some hints:

 1) a transformer has no set impedance; it reflects the load impedance back to the source as a function of turns ratio.  There are calculators on line. 

2) You lower the load impedance from the MM's typical 47k by adding resistors in parallel.  Some people solder resistors across RCA plugs and insert them into Ts at the preamp's MM In.  Others find or make a switchbox for this purpose.

3)The thumbrule for desired reflected impedance at the cart is to start at 2x the DC resistance and go up.

4) For a DL-103 (40 ohms) you want to start around 80 ohms.  A common turns ratio for use with DL-103s (and others) is 1x10.  A 47k MM input would reflect as 470 ohms; we need 8k, to reflect as 80.  A standard E96 1% value of 9.76k in pararallel will give you 80.1.  I usually wind up at about 100 ohms (Denon says >100); 12.7k yields 99.98 ohms.  15.8k yields 118 ohms...

A caveat:

The reflected impedances only work that neatly (factors of 10) for 1x10 transformers - there are square roots in the formula.

All vintage Denon SUTs are 1x10 or have that option via switch.

Sorry - one more:  That 2x impedance thumbrule only applies to stepup transformers.  If you are using an MC amp stage, it's 10x.  For the DL-103 that would be 400; some inexpensive phono preamps are fixed at 100 ohms or thereabouts.  You would need a soldering iron and a schematic to do anything about that.  My PS Audio Nuwave Phono Converter includes settings for 60, 100, 160, 600 and above.  The lower 3 don't sound good. - maybe "loose" and losing frequency extremes.  600 sounds best; I've lowered it to 400 with a switchbox but it didn't make much difference if any.  Above 1000 or so it started to "thin".

If

# of turns primary - Np
# of turns secondary - Ns
primary impedance - Zp
secondary impedance  - Zs

then

Np/Ns = square root of (Zp/Zs)

Zp = (Np/Ns) squared x Zs

Just Google "transformer impedance ratio" for the above formula.

for a 1x10 transformer, Zp/Zs = 0.1 so Zp = (.01)Zs

As you can see it is simple for 1x10.  You will want to use a scientific calculator app for any other values... it's still not THAT hard.

A coil (the MC) reacts differently to transformer loading.  For an RC  coupled amp we want the the input Z to be 10x (or more) the Z of the incoming signal to avoid distortion and losses due to the RC load that the amp input represents.  To put it very simply, if you matched the impedances you would drop (lose) half of the signal in the outputZ of the source.  Transformer action is not amplification, it is energy transfer and works differently.  A transformer would tranfer max power with a perfect Z match but this would lower the transformer's bandwidth (among other things).

Try it!