Yeah, at 10% tolerance, a 40uF could be in spec at 44uF, and a 47uF at as low as 42.3uF. And at a more typical 20%, there's even more overlap.
Since electrolytic caps are one of the highest failure components in an amp, why not use ones that are designed and tested for low impedance, 105 degree C temperature, and long life every time?
A 47uF/450V Nichicon VR series, their "standard" series of radials, costs $1.88 each at Mouser.
The 47uF/450V PW series costs $2.21 each.
Compare their specs - the PW is superior in every way, and will be more reliable due to its design and testing - well worth a mere 33 cents each!
http://products.nichicon.co.jp/en/pdf/XJA043/e-vr.pdf
http://products.nichicon.co.jp/en/pdf/XJA043/e-pw.pdf
Nichicon Filter Caps
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