Arrow lines crooked when using xymatrix frame around group of entries

The following does not yield squared arrow lines. The lines connecting the two lower blocks are crooked. Does anyone have any idea how to fix that?

Thanks,
Dave

usepackage{tikz}
usepackage[all,cmtip]{xy}
xymatrix {
        {scriptsizebegin{array}{ll}z\~\~end{array}}
    &
    &   *+++++[F]{G}    ar@<-9pt>[ll]
                        ar@{->}`l/0pt[ld]`[d]_{y}[d]
                        ar@{<-}`r/0pt[rd]`[d]^{v}[d]
    &
    &   {scriptsizebegin{array}{ll}w\~\~end{array}}            
                        ar@<-9pt>[ll]
\
    &
    &   *+++++[F]{K}    ar@<+9pt>@{->}`l/0pt[ld]`[d]_{x}[d]
                        ar@<-9pt>@{<-}`r/0pt[rd]`[d]^{u}[d]
    &
\
    &
    &   *++[F]{Q}
    &
}