Attenuator & Transformer for Moving Coil cartridge

No no no. If you’re serious about a high quality result, I would not advise placing any sort of attenuator in line between the tonearm and your phono stage. Unfortunately, the best choice here would be to try to find an SUT with a 1:10 or lower step-up ratio or a phono stage with more gain. 

15x ratio for a 0.6mV can actually work great, contingent upon the synergy of your cart to a particular SUT. Just try it and see what you’ve got. Pretty sure it stands at least a reasonable chance of beating a solid-state MC stage. Depends of course on the gain structure of your system and overload margins in the MM stage, etc. But don’t assume it can’t sound awesome. 

Ive used 16x and 20x SUTs with 0.5mV and 0.6mV, with superb results.

I'm not sure a 1:15 would be that much of a problem -- the XLPS-V3 appears to have plenty of overload margin -- provided your preamp is comfortable with the level presented it may work

A more problematic issue may be load matching from cartridge to the step up -- the MK10T has a listed input impedance of 18ohms and the Delos an impedance of 8.2 ohms. Lyra state "recommended load via step up transformer is 5-15 ohms" so the MK10T may work -- there's a lot more discussion on this topic here
http://www.rothwellaudioproducts.co.uk/html/mc_step-up_transformers_explai.html

Thank you guys for your responses. 
After reading all of these I have put aside the idea of of trying to reduce the output of the SUT, as the wrong approach.
I the next day or two I will actually try the Signet SUT with the Delos and will report back of my findings.

I should have said it myself; try it (the 1:15 SUT with no attenuation of its output).  You can't break anything.  I concur with the others that it might work OK.  Except I don't necessarily agree that a good MC phono stage is never as good as MM + SUT.  But that's not the question here.

Hello again guys;
I am so glad that I posted my question/apprehension on this forum.
After all the above replies , last night I decided to connect the MK10T transformer and hear for myself.
As I said at the start of this thread I already thought that Delos was an amazing cartridge and were enjoying it every night. However after connecting the transformer. the sound took on an entirely different character. The best way I can describe it is a much wider soundstage and so much more space between instruments and and individual singers being either forward or background. The midrange is much more liquid now.
The effects are so startling that last night I ended up listening to LP after LP until about 3 AM !

As a result of the 1-15 turn ratio of the Transformer the sound volume is ever so slightly higher than before. In fact this has the added advantage that there is not that much difference now between my CD player and the Analog set up in terms of volume (I don't have to continually change the volume whenever changing source, small point but it was mainly annoying).

Once again thank you guys for encouraging me to give this transformer a try and be so pleasantly surprised.