I've noticed that butlp produces pretty pronounced artefacts when the cutoff frequency is moving in any way quickly as the example below demonstrates. Obviously things are dramatically improved when kr = sr but at lower control rates all of Csound's other low-pass filters sound significantly better. I am wondering whether butlp is missing out on some internal interpolation.
I'm testing on 6.01 OSX10.6.

<CsoundSynthesizer>
<CsOptions>
-odac -d
</CsOptions>
<CsInstruments>

0dbfs=1
nchnls = 2
ksmps = 16

alwayson 1

instr 1
koct lfo 3,4,1
asig vco2 0.5,400,4,0.5

asig butlp asig,cpsoct(7+koct)

;asig clfilt asig,cpsoct(7+koct),0,2
;asig tonex asig,cpsoct(7+koct),3
;asig moogladder asig,cpsoct(7+koct),0
;asig bqrez asig/3,cpsoct(7+koct),1
;ahp,asig,abp,abr statevar asig, cpsoct(7+koct), 1

outs asig,asig
endin


</CsInstruments>
 
<CsScore>
</CsScore>
</CsoundSynthesizer>