Aligning equations inside 'cases' with individual text tags in each row and equation labels

Question

I am looking to produce an equation with the three objectives outlined in the image below. I'm unable to use \tag inside 'cases' and have been unsuccessful with other attempts. As described in the image, the three goals are:

1. to center the equation and align the equations from each row with each other,
2. put the texts in each row ("for case 1" and "for case 2") centered relative to the centered equation and the label for each row, and
3. have individual equation labels for each row (4a, 4b) whereby the number is obtained automatically from the natural order of other equations in the text (i.e., the number is not hardcoded). But if needed, hardcoded is OK.

Thank you in advance.

MWE:

\documentclass[12pt,a4paper]{article}
\usepackage{cases}
\usepackage{subeqnarray}
\usepackage{longtable,tabularx}
\usepackage{amsmath,mathrsfs}
\usepackage{mathtools}


\begin{document}

Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla bla bla bla bla: \begin{equation} A =  \begin{cases}  \begin{aligned}
      0.55 B~, \quad\quad \text{\rm for~case 1}~,& \\
      0.75B~, \quad\quad \text{\rm for~case 2}~.& \end{aligned} \end{cases} \end{equation}

where Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla bla bla bla bla bla.

\end{document}

Answer 1

With empheq:

\documentclass{article}
\usepackage{amsmath,empheq}

\begin{document}

\begin{subequations}
Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla 
bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla bla bla bla bla:
\begin{empheq}[left={A=\empheqlbrace}]{alignat=2}
  &0.55 B, &\qquad\qquad&\rlap{for~case 1,} \\
  &0.75 B, &&\rlap{for~case 2,}
\end{empheq}
where Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla 
Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla bla bla 
bla bla bla.
\end{subequations}

\end{document}

1. \rm has been deprecated for about 30 years.
2. ~, has excessive space; maybe \,, (not that I'd use it).

On the other hand, I don't see why moving the labels too far.

\documentclass{article}
\usepackage{amsmath,empheq}

\begin{document}

\begin{subequations}
Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla
bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla bla bla bla bla:
\begin{empheq}[left={A=\empheqlbrace}]{alignat=2}
  &0.55 B, &\quad&\text{for~case 1,} \\
  &0.75 B, &&\text{for~case 2,}
\end{empheq}
where Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla
Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla bla bla
bla bla bla.
\end{subequations}

\end{document}

Third possibility, but I see no improvement; actually, I think it's worse typesetting.

\documentclass{article}
\usepackage{amsmath,empheq}

\begin{document}

\begin{subequations}
Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla 
bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla bla bla bla bla:
\begin{empheq}[left={A=\empheqlbrace}]{alignat=2}
  &0.55 B, &&\rlap{\quad for~case 1,} \\
  &0.75 B, &&\rlap{\quad for~case 2,}
\end{empheq}
where Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla 
Bla bla bla Bla bla bla Bla bla bla Bla bla bla Bla bla bla bla bla 
bla bla bla.
\end{subequations}

\end{document}