Mixing different tubes in a preamp

Does anyone have any experience with the results of mixing tubes from different manufacturers in a preamp? I have a BAT VK-31 which uses 6 6922s. I replaced the stock Sovteks with what I happen to have on hand, 2 pairs of Telefunkens and 1 pair of Mullards,, distributing them so that the left and right channels are symmetrical. The sound is fine to me, but I'm wondering whether anyone has actually performed this type of substitution in his or her own preamp, whichever brand, and can recount the results or would otherwise care to comment. I realize that the optimum arrangement would be 3 matched pairs from the same manufacturer, but I'm trying to avoid the expense. Thanks.