Hi Rene: Normally I wouldn't recommend a step-up transformer with the Delos unless your phono either has low gain or good overload margin.
The 0.6mV output from the Delos is high enough that it caused clipping when we matched it up with a Nagra BPS, which (according to Stereophile) has 51dB gain in fixed-coil mode and 62dB with the built-in stepup transformer in the circuit. Jumpering out the transformer cleared up the situation. I presume that the 9V power supply of the BPS is to blame, and that using a different phono stage with a higher power supply voltage would avoid the problem.
If you use a transformer, I suggest 1:10 ratio, loading at the secondary, and very short, very low-capacitance cable connecting the transformer to the phono stage.
Boosting the output from a Delos with a 1:10 ratio will present your phono stage with a 6mV input, which should be quite comfortable for all MM and MI-level phono stages.
If your phono stage has 60dB gain or more, you won't need a transformer (unless you have very low preamp/power amp gain, or very inefficient speakers). If your phono stage gain is in the 40-some dB range, you will probably need a headamp or stepup transformer. Again, your results will be affected by preamp/power amp gain and speaker efficiency, but likely not enough to let you get by without the extra gain stage.
The situation is more unclear with phono stages in the 50-some dB range. Here I don't have any firm recommendations, other than suggesting that you compare with and without a transformer or headamp. Perhaps you could first find a vintage unit of low price and the right properties, then change to a higher-quality unit once you've verified that things work.
FWIW, we also make the Erodion (http://www.lyraconnoisseur.com/Products/Products_Analog/Erodion/erodion.html), which is a stepup device, but this has a 1:20 ratio, so I wouldn't recommend it for the Delos unless your phono stage gain is in the low 40-some dB range.
hth, jonathan
