Small or big tubes/valves for pre amp ?

No, don't care for the GTA/GTB's.

I ran a single-gain-stage preamp with all brands of NOS 6SN7s and loved them (some more than others). But that design easily revealed microphonics. Using Herbies tube dampers sometimes helped.

@rodman99999
My whole point is ,The Sylvanias don’t have near the bass of the Ken Rads or Tung Sol 6SN7. Also Andy with Vintage Tube Services confirmed this for me . He actually told me that any of the Sylvania 6SN7s do not do Bass well .I agree with the article about mixing them. The main problem is that a Very High percentage of Tung Sol and the Ken Rads are Micrphonic. these are the only two 6SN7s that have great Bass.I have tried many many different 6SN7s. SO using a 6F8G with the 6SN7 adapter in place of the Ken Rad or Tung Sol , works great and gives you way more options
@mulveling
I agree with you 100%

Given the number of circuit designers, over the decades, that have chosen to utilize both nine pins(ie: 6DJ8 iterations) and octals(ie: 6SN7s), and the excellent results, that have been attained with both choices, has led me to believe that blanket statements are worthless. They only serve to prevent some from experimenting on their own, to find their personal preferences. Regarding Sylvanias and, "having Bass": I don’t want my tubes to, "have" anything of their own, but- to pass what’s presented them, without leaving their own footprints. The author of the 6SN7 comparison, made this statement(his first sentence): "Let me stress that the optimum tube for your system will depend on your equipment and how your ears judge the tube’s ability to reproduce live music." It seems some miss(or choose to ignore) those very salient(and universal) points(circuit variations/subjectivity).

@mulveling-  Mixing tube types in a circuit, to reinforce the positives of each, doesn’t ONLY apply to 6SN7s, but works in most any application.

Best preamp I have ever heard uses the 6N1P