My cables are 30 and 40 cm between source and passive attenuator and 2.30 m between the latter and the power amp. No problem at all and great sound. I got a similar sonic balance with an active preamp (Prehead MkII) -- which proves that this length isn't critical in my setup.
What's important apart from cable length and capacitance is the ratio between source output impedance and power-amp input impedance. It should be as high as possible. My main source, the Bel Canto DAC2, has 20 ohm, the Metaxas Solitaire has 100 kOhm, making a ratio of 2000.That's a huge number. But I also got good results with my Hitachi FM tuner (2 kOhm) and my BEL 1001 power amp (20 kOhm), thus a ratio of 10, with the same cable lengths. But this could indeed be a little close for a perfectly neutral sonic balance.
In my experience nothing beats passive amps in an electrically favorable system when it comes to signal accuracy and sonic purity. In an uncritical environment all «improvements» resulting from an active preamp are in fact euphonic colorations. But one thing must be taken into consideration: the passive variant can be too revealing in certain cases.
