I owned a Tambaqui for a short while until a few weeks ago. I liked the sound better when using an outboard preamp. I also played with the output voltage, which you can do through the app but I cannot remember which setting I liked best, maybe the top position (6V?).
You should definitely call them but I believe TMR is actually a Mola Mola dealer. The only way you will be able to answer these questions is to try one for yourself with/without an outboard preamp, and trying the different configurations in your own system. Below is the return policy posted on TMR's website - a 60-day trial is hard to beat IMO.
New product lines for which TMR is an authorized dealer carry a 60-day satisfaction-guaranteed return policy. Return shipping costs are at the expense of the customer unless the product is defective. Full manufacturer warranties are in effect for these products.