'outboard' high end dac's

Can anyone recommend an 'outboard' dac to be used in conjunction with an Oppo 105d (sabre 9018 chip), which might improve on the somewhat thin sound of the Oppo, improve on the (decreased relative to the Oppo) clarity noted when a Denon 1713ud (BB chip) was A/B'd with the Oppo, and decodes DSD and PCM formats?
Thanks