As I write this, I am listening to my $1100 Class A Schitt Mjolnir preamp with a $800 Class A Schitt Aegir amp and a Schitt Yggi+ LIM DAC with a RAAL CA-1a headphone. I am listening to the group Sault and it is incredible. I need warmth in this setup.
This preamp is going work well with the KRELL DUO. Forget about the Tambaqui's contribution to the sound. That is not going to determine your sound as much as the KRELL. I personally would not use the Mjolnir with Blades since the other 2 preamps I mentioned are not warm, but the Mjolnir will do what you seek with added warmth. This preamp also has a 15-day home trial. It sounds great with Class A amps like the Aegir and I am sure the Class A KRELL.
I know the sound of the KRELL XD well since I owned 2 of these pieces and I am debating trading in my big amp for the new KRELL KSA i400. Your KRELL is going to dominate your sound, not the Tambaqui. Seek a preamp that matches the amp.