The RTL2 mk1 is a bit bright in standard form, so I wouldn't increase the tweeter level if I were you. Changing the cap adjusts the frequency the tweeter comes in. Changing to 4.7uF brings the tweeter in later and tames a slight rise at 2KHz according to the chap who designed them (Clive Gibson). I just found the remaining sparkle a bit detached from the midrange when set this way, although just using the one, higher value cap without the second smaller value one is fine I was told.
If you use a tweeter like the Scan which has a much wider bandwidth, you may care to use the 4.7uF cap instead of the 5.7uF one, as this "should" restore the tweeter roll-in characteristics I think.
The maplin cable I suggested is a general instrument cable, not a "speaker" one.
http://www.maplin.co.uk/Module.aspx?ModuleNo=6200
The link above is for the red one, also available in black.
The QED Micro is too low in gauge to be regarded as a serious HiFi cable, no matter how good the copper inside. It's the kind of stuff we were throwing out thirty years ago in favour of heavier gauge wires. I appreciate things have gone mad regarding cables, but the stuff linked to above is fine and not expensive at £1.78 per meter (red plus black). It's flexible enough to to work inside a speaker cabinet too.
Listening to speakers back to back can be a minefield, especially on less expensive models where various compromises are all too obvious. The RTL2 mk1 model was quite natural in balance apart from the slightly bright treble register and the bass went down further and cleaner than many other small inexpensive speakers of the period. This doesn't mean to say that they could be improved though......
As I stated earlier, the better finished RTL2 mk2 (SE) had very little treble at all, to make it compete with the AE109. The treble shelved down horribly, but could be partly corrected by decreasing the tweeter resistor from 5 Ohms approx to 1.5 or 1 Ohms (I can't remember exactly as it was so long ago).
To be honest, I'd try to find some Studio 1 M's. They're lovely with a smooth balance (despite the metal cone bass/mid) and take over and expand where the RTL series leaves off.
Apologies for being a bit vague with some of these tweaks, but it was nearly twenty years ago now and the nineties was a decade of major personal change, grief, joy and growth for me, so these latter memories take precedence in my mind over the more mundane (by comparison) audio ones.