Gulp + Babel + Webpack + Karma: Use sourcemaps for stacktrace - angularjs

Question

In an AngularJS 1 project we use Gulp to compile the project with webpack and test it with Karma. We cannot get sourcemaps to work as expected.
I found the obvious plug-in (https://www.npmjs.com/package/karma-sourcemap-loader) but this seems to exist only to use sourcemaps while debugging karma tests, NOT to display a usable stacktrace.
I also found https://www.npmjs.com/package/karma-source-map-support which seems to be exclusive for browserify.
Am I missing something here? Has anyone gotten this to work?
Appended info:
This is the unusable output (note index.module.js, the webpack compiled file, which has inline sourcemaps):
PhantomJS 2.1.1 (Mac OS X 0.0.0): Executed 629 of 629 (1 FAILED) (6.663 secs / 6.531 secs)
TOTAL: 1 FAILED, 628 SUCCESS
1) should A B not be A B
TrajectDetails Controller _aMethod()
Expected Object({ a: Object({ x: [ 'A', 'B' ] }) }) not to be Object({ materieel: Object({ x: [ 'A', 'B' ] }) }).
.tmp/serve/app/index.module.js:93820:42
[11:02:26] 'test' errored after 9.37 s
[11:02:26] Error: Failed 1 tests.
This is how my karma config looks:
var preprocessors = {};
pathSrcJs.forEach(function (path) {
preprocessors[path] = ['babel', 'sourcemap'];
});
pathSrcHtml.forEach(function (path) {
preprocessors[path] = ['ng-html2js'];
});
var configuration = {
files: listFiles(),
preprocessors: preprocessors,
singleRun: true,
colors: true,
autoWatch: false,
ngHtml2JsPreprocessor: {
stripPrefix: conf.paths.src + '/',
moduleName: 'xx-xx'
},
logLevel: 'INFO',
frameworks: ['source-map-support', 'jasmine'],
browsers: ['PhantomJS'],
plugins: [
'karma-phantomjs-launcher',
'karma-coverage',
'karma-jasmine',
'karma-ng-html2js-preprocessor',
'karma-spec-reporter',
'karma-babel-preprocessor',
'karma-sourcemap-loader',
'karma-source-map-support'
],
reporters: ['progress', 'spec'],
};
config.set(configuration);
Webpack config is like this:
var webpackOptions = {
watch: watch,
module: {
loaders: [{test: /\.js$/, exclude: /node_modules/, loaders: ['ng-annotate', 'babel-loader']}]
},
output: {filename: output},
devtool = 'inline-source-map'
};
var sources = [path.join(conf.paths.src, source)];
sources.push(path.join(conf.paths.src, '/app/**/*.spec.js'));
return gulp.src(sources)
.pipe(webpack(webpackOptions))
.pipe(gulp.dest(path.join(conf.paths.tmp, '/serve/app')));