The LL1540 is a relatively high impedance transformer, so it's not really ideal for this service, but it's not off-the-wall either, largely because your attentuator doesn't load it excessively. Of course, the output impedance is essentially the value of your series resistors in your attentuator, so don't use long interconnects.
If you use a higher mu tube, you can use a transformer in stepdown and use lower value series resistors and attenuator, too, to get the output impedance down.
Kevin Carter
K&K Audio
www.kandkaudio.com