Not sure if they from Vishay I bought them frm Element 14 its Blue in color well it simply state Powertron in their list.
I no longer use them, I use the Mills MRA12,s (I've tried Ohmite too) & mounted it outside the chassis, some recommend using a pair in parallel & I stopped using Diode Bias for the reasons Andy mentioned!
I used two 10R in parallel to make 5R and added 1.2R Welwyn to make 6.2R in one of the 26 stages. You could use three 27R in parallel also depending on your needs.
5X51R in Parallel, this will give you a buffer of 20 Watts (50W). I did build a 26 a long time back with these, that's how I did it.
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Just to add to my previous reply, I found the Miflex KPCU 1 to be better in just about every way to FT3, more Dynamic, they add life with more speed, better Timbre, it's very noticeable with Piano & Cello, low level detail is much clearer too, the FT3's have about 200 hours on them & the Miflex a lot less, I had to go back & do a comparison between the two after what has been mentioned here, I've tried other expensive types from Mundorf, Auricap, Clarity Cap & Jupiter, all with different results, but these Miflex will be staying in, they are also reasonably priced for a copper cap, I think they are worth every penny for SQ, there's no edge to the sound either, which was the reason I went away from Diode Bias.
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That should be plenty, especially if you mount them outside the chassis, I'm using 1 mounted outside for 01a (250mA), they still get warm
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I've got the 630V as the 250V weren't available when I got them. The Jupiter were Copper Foil in Wax, the Miflex are Copper foil, Paper/Polypropylene in oil, the Jupiters were good but for the cost a sideways step if that & for more money, the flat stack that I had I believe are no longer available, I've heard claims that Miflex have outdone the best in some systems, I can't vouch for that, but they are going to be similar to other copper Foil types.
Rule of the thumb: if you don't have heatsink, choose at least three-four times greater wattage resistor than dissipation.
Also if you mount the resistors inside the chassis, that's a lot heat where you don't want it which causes drift plus early component breakdown
Nice build, what cable are you using between PSU & Preamp, also why did you need to use extra filtering at Gyrator & source follower & did it make much difference?