It depends on the output of your cart and the total gain of your system. KAB has a gain calculator on their web site.
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EAR makes some of the best phono SUT's on the planet. Not sure why you'd try anything else. In fact, your 868 may have a SUT already in the signal path which is what might be causing the hum... a sut connecting to a sut...
I'm with Swampwalker, what cartridge, output are you using?
I have a EAR MC4 SUT...it's a killer good SUT. This might be one of the very few instances where the wonderful bob's devices sut is a step down from the sut you already have built in...
maybe one tweak to far ;-)
Hi I found a review on your EAR 868. It does indeed use transformers already in the phono stage. "...the linestage gain is 17dB, the phono gain a variable 50-80dB, and the output impedance a relatively low 600 ohms. The 868 doesnt invert phase. Also, rather than using another set of tubes for a third gain stage in the phono section with its attendant risk of noise, EARs legendary designer, Tim de Paravicini, chose to handle that with custom-designed step-up transformers, with selectable primaries of 10/1, 20/1, and 30/1." This is quoted from: http://www.soundstage.com/equipment/ear_868.htm
So seems like you have a massive amount of gain..up to 97 db (17 in line stage and up to 80 in the phono)
Have you played around with your loading via the dip switches? These switches are actually selecting different windings on the transformer...giving you different gain. (Similar to my EAR MC4 SUT)
Those Cardas Ruby's are nice cartridges...What table and arm are you using? I've heard the EAR Pre and Amp before..that gear is VERY good...and likely not a source of improvement...unless you want to spend very big bucks...