BAT vk 31 bat pack as an upgrade

Has anyone ab'd a bat pre with and without the batpack upgrade?This is one piece i have kept stock.Is it worth an upgrade or trade up? This things great but can I get a big return or just a suttle upgrad? vs the cost.Jusy wondering thanks.
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In the same spirit as rickw062 query, does B.A.T. make an excellent integrated amp? Which model?
Anyone own a BAT integrated amp?