Rajesh,
it was A hA indeed, or more of 'ch ch ch.. inducing'. One middle-aged gentleman on the next aisle one row back just about lost it in rapture when the sisters announced the abhang and again upon the first nano second of them singing that abhang!! That alone is hundred times more in intensity than a few of my 'A hA.. ch ch ch' moments in other places. That gentleman also had many non-rapturous 'A hA' moments throughout the concert.
My own were a couple of places in Vazhi maraiththirukkude where they sang with some emotion in their voice ( not done artificially or overdone ), and kA vA vA where they held on plainly and steadily to a 'close enough to Sa' note beautifully before resolving to Sa.
There were a couple of places in kIravANi alapana as well which attracted my attention. That is more because I look for some romanticism in the interpretation of this raga (hard to explain, that is the closest word ). Not that this raga is normally sung that way since the songs in this raga adapt it for different emotions, but I always felt this raga has that potential. In their alapana, I saw some glimpses of that possibility.
Overall, their impeccable swarasthana suddham, singing to their strengths, voice dynamics, just about maximizing the 'power of duo' to good effect carried them through.
If I nit-pick, it will be their kamAs. There were definitely places where the well known phrases are sung where kamAs was alive and well, but there were quite a few stretches where it was quite generic. Especially the thAnam.. May be it is difficult to bring out kamAs in thAnam singing.
Their kalpanaswaras for kamAs can be better, though they made up for it plenty with their sheer energy and enthusiasm in their singing aided by an equally adventurous and boisterous mridangist.. Being a duo, it helps a lot in that energy department and they can string together long stretches of kalpanaswaras at high speed since they alternate even at that high speed ( not that it is easy and they execute it well ). There was also a bit of novelty ( for me, may be not for you all, I do not attend a lot of live concerts ). In that high speed kamAs kalpanaswara section, they sang the word 'raghuvamsa' at the right place in the thala cycle where raghuvamsa would have been. Hope you get the picture, otherwise I will explain further. It all added to the excitement and entertainment value just on the mechanics of it and the overall sound environment it created. But kamAs itself was not the focus during those sections.
; if it was the latter then it must have been the ghost of Harimau 