Emotiva XSP-1, fully-balanced, differential, reference preamp, 5 year warranty, $899...
Keep in mind that when using a tube preamp with a solid state power amp, which is what I presume is in your active speakers, impedance compatibility tends to become a particularly important issue.
Many of the BAT preamps, for instance, have output impedances which rise to the 4K to 5K area at 20 Hz, as indicated in JA's measurements in Stereophile's reviews of several of those preamps. Depending on the deep bass extension of the speakers, bass performance may start to become marginal with those models if the input impedance of the amplifier in the speakers is significantly less than around 40K.
And as indicated here the BAT VK-3iX has a balanced output impedance ranging "from a very high 14k ohms at 20Hz to 1850 ohms in the midband and above." I would not use that preamp with an amplifier having an input impedance of less than 100K, and even that number, being about seven times the 20 Hz output impedance, is a bit marginal (ideally that ratio should be 10x or more). The majority of solid state amplifiers have input impedances that are much lower than 100K.
The feed is balanced so a balanced pre would be best although RCA to XLR is doable.Keep in mind that if you do this with a simple adapter or adapter cable (as opposed to a transformer or an active device) the load impedance seen by the preamp may be a considerably lower number than the specified balanced input impedance of the speaker. Perhaps a factor of two lower.